Hello,
There's Rational
Rose. You can download it for free from
www.rational.com I found this quite hard to use at first but it's pretty good.
Theres a program called Visio but from memory you need the professional edition to be able to draw UML diagrams. This is does not generate any code but is easy to use for diagrams. This is a Microsoft product.
There is probably a list full of free ones so do a search for CASE tools on google and see what it comes up with.
Ian