I personally use the MIT License (as defined by the OSI; in FSF terms, it would be the "Expat" license) as my default license for most things. My personal rule of thumb is that a software license should readably fit on a single screen whenever possible (I got sick of folks like Apple and Microsoft and their million-page EULAs), and most "copyfree" licenses, like the MIT/Expat (and MIT/X11), BSD, ISC, etc. licenses fit the bill rather perfectly.

I've tended to avoid copyleft licenses due to the potential complications caused by license incompatibility when trying to combine code with other code; since I'd prefer my code to be as useful to as many open-source projects as possible, I consequently prefer something as relaxed as possible.

My one exception (other than when required to make an exception by an employer or client, of course), however, is when I'm writing Perl code (particularly Perl code that's meant to be reused, such as something on CPAN); in these cases, it's standard convention to use either "the same license as Perl itself" (Artistic License / GNU GPL dual-license) or the Artistic License 2.0 (or later), so I just stick to convention.

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Those are my needs, however. Everyone has different needs for a license.

If your priority is ensuring that your code doesn't make its way into proprietary projects, the most surefire way to do this is with a "copyleft" license. The GNU General Public License (version 2 or later) is probably the most common such license. When I use the GNU GPL (aside from with Perl packages), I tend to use version 2 with an "or any later version" statement; version 3 includes anti-TiVOization language that - while useful if I'm an extremist - is a bit excessive for my tastes. Other options here include the GNU "Lesser" General Public License (basically the GPL, except that it allows non-free software to dynamically link against your software; useful for libraries, for example), the Mozilla Public License 2.0 (allows integration with differently-licensed software, so long as the MPL'd parts remain MPL'd), and the Artistic License 2.0 (derived works may be distributed either under the same license (to either everyone or just the original author, at the new author's discretion) or under a different license but forbidden to use the original name or interfere with the installation of the original version); the MPL2.0 and AL2.0 are both compatible with the GPL (though the MPL has an optional "Exhibit B" that can disallow that compatibility), and are better classified as being "partial" copyleft licenses (in the sense that they aren't as strict as even the LGPL).

If your priority is instead just to make sure you're credited for your work, pretty much any non-public-domain-dedicating free software license will do in practice, though the BSD license is the most explicit about this in the copyfree camp (the 2-clause or 3-clause is recommended; the 4-clause is frowned upon). The ISC and MIT (of the Expat or X11 varieties) are also suitable here, though they're not as explicit about attribution in binary distributions of derived works.

If you're concerned about patents, the Apache License 2.0 and GNU General Public License version 3 provide some protections regarding patents that might be useful. The Apache License has some other nice features as well (trademark protection, proper attribution, non-copyleft, default contribution agreement, etc.).

It's difficult to provide a more specific suggestion without clarification regarding what, exactly, about your code you intend to "protect".