The free version of Oracle10g is called
Oracle Database 10g Express Edition
Links to download the Windows and Linux versions are at the top of
http://www.oracle.com/technology/software/products/database/oracle10g/index.html There are some limitations:
1. Express Edition is limited to a single instance on any server;
2. Express Edition may be installed on a multiple CPU server, but may only be executed on one processor in any server;
3. Express Edition may only be used to support up to 4GB of user data (not including Express Edition system data);
4. Express Edition may use up to 1 GB RAM of available memory.
If this is for evaluation purposes and not deployment, I believe Oracle allow you to download the full database product with none of the above limitations. You can download that from the same page.