A Myriad Reasons Giver here...Check the above link too...
Public Domain APIs - Any Java public apis are part of the public domain, .NET apis are proprietary and can open the door to a law suit.
Standard Library Source Code Availability - Java source code for the core libraries are available in every J2SDK distribution, .NET sources can only be seen by resorting to illegal means.
dotNet Purity Is A Myth - Java promotes 100% pure Java libraries, for .NET purity is nothing more than a myth.
75% Of Enterprise Software Development - Avoid becoming one of the 25% of "use-less" employees.
Preferred Platform for Implementing WebServices - Despite billions spent by Microsoft in marketing, surveys continue to reveal that Java is the preferred platform when it comes to Web Services.
Superior Platform for Web Development - ASP.NET is a poorly designed and crippled framework as compared to the richness of frameworks found in Java.
Write Stored Procedures using Java - Most relational databases support writing of stored procedures in the Java language. There has yet to be a production release of a database that supports any .NET languages.
An Abundance of Experienced Practitioners - Nobody seems to know how to write .NET programs well and that's giving .NET a bad name! A pretty lame excuse I must say!
Supportive Open Source Communities - Open Source communities that support distributed development are a plenty in the Java world.
Proven Security - 2 Years after Trust Worthy initiative is launched and we collectively lose $55 billion last year.
No Lizard Brain - .NET programmers continue to struggle with the complexities of a hybrid managed/unmanaged environment.
More Languages - The JVM is more "common" than the CLR.
Smaller Runtime Download - You can't run your app if you don't have the runtime.
No Mandatory Upgrade Fees - 3 things a Microsoft shop can't avoid: Death, Taxes and License 6.
More Vendor Choices - .Net is a marketing program. Java is a Market.
Industrial Strength Collection Classes - The .Net libraries look like they were designed by high-school students, First year CompSci? students at best.
FutureProof - The way to ensure your return on investment (i.e. ROI) is that your choice of platform doesn't get obsolete in 5 or even 10 years. Avoid the Microsoft upgrade treadmill!
Larger Talent Pool - Majority of Universities not only teach but require knowledge of Java. That's a big talent pool that you need to consider before you off-shore your project to a different time-zone.
More Contributions From Researchers - Research institutions and universities have consistently provided innovative research not only built on top of Java but also contributing to Java.
US Government Approved - Guess where the billions of dollars spent on the U.S. government's IT renovation is going to?
NoEvilTypeCoercion - Some C++ constructs are meant to be entombed forever, .NET resurrects them with disastrous consquences.
More Mature And Robust O/R Mapping - You can't beat the wealth of O/R mapping solutions found in Java.
Superior Coding Tools - Like having your own personal Java fairy dancing through your code, anticipating your every thought and keystroke.
Sane Coding Conventions - I don't know what's worse Hungarian notation or .NET coding conventions.
Higher Paying Jobs - Somehow you've got to afford those skyrocketing housing prices don't you?
Favored for Implementing P2P - Gnutella and JXTA, anything else legally more pervasive?
PureJavaRDMS - Can't beat the ease of installation when the RDMS is Java based and packaged with the application in a .zip file.
More Exception Handling Options - .NET has no analog to the throws clause in method signatures.
Better Structured Diagramming Frameworks - When going beyond forms and windows, Java can't be beat.
Reusable Open Source IDE Frameworks - Why re-invent the wheel? Start building your killer GUI application on top of killer IDE frameworks.
RobustParserGenerators - Want to build a new language, well you'll need some robust parser building tools.
AspectOrientedProgramming - The next advance in modular software development, get a head start by using Java.
PureJavaWebServers - Customizations and extensions are easier in a web container that's built using the same language as applications. Furthermore, managed environments support better reliability and security.
OpenSourceJavaCompilers - Your tools have got to be able to parse the code before it has any chance in understanding it.
Distributed Caching - Sometimes embarassingly parallel applications aren't the only things that you need to scale.
MultipleReliableMessagingChoices - Java provides more choices for the backbone that integrates the Enterprise.
Faster Development Turnaround - Incremental compilation is unavailable in the .NET environment.
Lightweight Persistence - Sometimes a relational database (RDMS) has too big a footprint.
Open Community Process - How does one contribute to the specification of standards?
Hardware Accelerators - Performance boosting hardware.
More Flexible Licensing Options - Ultimate flexibility in licensing.
EmbeddedDevices - Java inside small packages.
Faster Virtual Machines -
WholeProgramOptimization - Please sir may I have a linker?
Comprehensive RDMS Driver Support - Can you find a ADO.NET driver for an open source database?
Superior Code Analysis Tools
NetworkingSupport - Why is MSN managed by a Java based tool?
More Garbage Collection Options
Better Web Services Interoperability
Better Domain Specific Languages Support
Simple Side By Side Execution
More Business Rules Engines
Better Business Process Management
Sixty Four Bit Support
Millions Of Java Phones
Garbage Collect Classes - The only way to unload MSIL code is to unload an entire application domain.
More Alternative VM Implementations
Hard Realtime Capabilities
Cross Platform Language Integration
More Extensive XML Support
Better Support For Dynamic Distributed Systems
Superior 2D Drawing
Better GUI Framework - Why isn't VS.NET written in a .NET language?
No Anti Open Source Agenda
Standardized Portal Frameworks - Standardized ""Integration at the glass"".
RunInIntepreterMode - ""We're just not optimized for interpreting""
More Semantic Web Research
Leads In Software Process Best Practices
Better Concurrency Utilities
More Multicasting Libraries
Superior Refactoring Tools
Higher Demand Therefore More Jobs
Faster And More Reliable Regex
SuperiorBuildEnvironments - A .NET practitioner's concept of a build is F7.
Embarassingly Rich Information Sources
More Open Source Projects
Affordable Industrial Grade IDEs
Standardized Enterprise Connectivity
MorePDASupport - Why limit oneself to a single PDA brand?
OpenTechnologyRoadmap - .NET is like a five year plan in the former USSR: You know it doesn't actually make sense or help anything, but if you live under it, you're certainly not going to say anything negative about it.
JavaCard - JavaCards? are becomming the preferred method of keeping tabs on your citizenry or customers. If Microsoft ever co-opts this technology then "1984" will become more than just a paperback novel.
Complete Open Source Stack - Open Source code visibility spanning all layers of an application.
NonStopServers - .NET not fault-tolerant enough for Hewlett-Packard-Compaq?
Out Of This World - Java runs on other planets, .NET has yet to leave Terra Firma.
Better Unit Testing Support - More extensions and comprehensive IDE support
More Identity Management Solutions - Can you trust Microsoft to keeping your customer's identity secure and available?
Most UML Tools Implement In Java - Ever wonder why the best UML tools are implemented in Java?
More R&D On Intelligent Agents - Java is the preferred implementation platform for Intelligent Agents.
Easy Rich Client Deployment - No-Touch development was shaky.
Lower Cost for Massively Parallel Systems - How much does it cost to deploy a .NET application on a platform with 10,000 servers like google?
More Profilers - Profilers mitigate the risk of not finding the root cause of show stopping bugs.