I got a little curious to see what libraries other indie games were using. So I opened my Steam folder and took a quick look.

• Osmos uses OpenAL, FreeType, libvorbis, libogg, libcurl
• Super Meat Boy seems to be using its own “Tommunism Engine”
• World of Goo uses ODE, SDL, parts of the PopCap framework, TinyXML, irrKlang, libcurl
• Machinarium is a Flash game. Uses funny filenames for the data, by the way.
• Aquaria uses ZLib, SDL, libvorbis, libogg, Lua, FreeType, FMod and more
• Braid seems to be using no 3rd party libraries beyond DirectX
• Revenge of the Titans uses LWJGL and Java

Some more findings:

• Torchlight uses Ogre3D and CEGUI
• Minecraft uses LWJGL and Java
• Family Farm uses Boost (serialization, threads), TinyXML, OgreMax, ParticleUniverse, cURL, Google Breakpad and wyUpdate for updating the Windows version