Here are some licenses I've been reviewing lately:
GPL:
http://www.gnu.org/licenses/gpl.txt
If I understand this license correctly, once the work is released and licensed under these terms, all derivative work must also be licensed similarly, if the work is distributed in compiled form.
LGPL:
http://www.gnu.org/licenses/lgpl.txt
The lesser GPL is similar, but it allows for derivative works and code modifications to be kept private, even if the original work is distributed in compiled form.
This made a big difference when software had to be compiled and distributed before use, but now that most software can be used without being compiled or distributed, the two licenses are pretty much the same.
Hence the Free Software Foundation (FSF) is working on a third version of the GPL, but I've heard that they are having a lot of troubles with it:
http://gplv3.fsf.org/gpl-draft-2007-03-28.html
The Affero-GPL (AGPL) is a GPL-compatible license that tries to bring the GPL into the internet age by requiring websites that use software licensed under the AGPL to offer the source software as a download link to all website visitors:
http://www.affero.org/oagpl.html
The FSF is also writing a new version of the AGPL, which looks to be even better:
https://www.fsf.org/blogs/licensing/2007-03-29-gplv3-saas
There are many other licenses, but most of them are not appropriate for what I'm trying to accomplish with PBooks, Private-Wiki, or Update.
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