zlib License
A short permissive licence that, unlike MIT, doesn't make you credit the author in the finished product.
Jean-loup Gailly and Mark Adler wrote it in 1995 for the zlib compression library — the code that makes PNG and gzip work. It has three conditions: don't claim you wrote the original, mark altered versions as altered, and don't strip the notice from source distributions. An acknowledgement in the documentation "would be appreciated but is not required."
That last clause is why game developers love it: nobody wants to cram a dozen licence texts into the credits. SDL, Bullet and LÖVE all use it. It is OSI-approved and GPL-compatible, and belongs to the small club of licences short enough that people actually read them.