gtatools is a collection of cross-platform (currently officially Windows and Linux, but should run on other UNIXes without major changes) libraries and tools for working with the files of the GTA game series. Support is planned for the GTA III trilogy (III, Vice City, San Andreas).
WARNING: gtatools is currently highly experimental (read: enormously bugged, poorly tested and only partially implemented). Use it at your own risk. I mean it.
The source code of gtatools is hosted on GitHub. The git repository code should be considered highly experimental.
You can find a guide for building gtatools here.
You can find some tips for using gtatools here.
These packages include most of the external libraries needed to compile gtatools from source, prebuilt for a particular platform.
Note that these packages do not include Qt. The Qt SDK has to be downloaded separately if you want to build gtatools-gui.