As you can probably tell from my email address, I'm a free software developer (here's some of the stuff I wrote). For information on the "free software" philosophy, have a look at the FSF's pages. Free software and Open Source are related concepts.
I release most of my code under the GNU GPL (unless I'm contributing it to another project which uses a different license, in which case I'll use the original license). This is just because I like the protection which the GPL affords; if you're looking for a license for something you wrote, I'd suggest that you consider as many as possible, including at least:
- The GNU General Public License, version 2. It's customary (but not required) to put a note on your code which allows "version 2 or any later version" to be used; the FSF are currently working on version 3, which should resolve problems with the definition of "derived work".
- The GNU Lesser General Public License (formerly the Library General Public License), version 2. This is similar to the GPL, but allows linking to non-GPL-licensed code.
- The Artistic License.
- The Mozilla Public License.
- The original BSD License. Many users these days remove clause 3 (which requires contributors' names to be included in advertising material).
- The X11 License.
- And, of course, you can release code into the public domain, in which case you need to explicitly mention it in the documentation.
If you're writing something that's not a computer program, but you want to apply the same sorts of licensing terms, here's some of your options:
- The GNU Free Documentation License. (This is rather specialised towards software documentation, though.)
- The Open Content family of licenses.
- The Creative Commons licenses. These are particularly neat because they let you build your own license with the features you want to include -- for instance, their license "kit" allows you to build reasonable clones of the GPL or BSD licenses.
Further information about free software licenses can be found on Debian's site (which includes the Debian Free Software Guidelines), on the OSI's site, and on the GNU licenses page.