s3pi library by Peter
This is an open source library to support people wishing to make tools for The Sims 3. It is written in C# under .Net 3.5 and contains wrappers that implement methods to read and write package files, unpack packed files, and interpret several filetypes.
The library has moved to .Net 4.
We are always open to programmers to contribute wrappers written to the specifications of S3PI. Details of how to do this, and how to use the library in your own tool, can be found here: http://s3pi.sourceforge.net/.
The library documentation is now included (s3piHelp.chm) in the .7z package for the parts that are covered with doc-comments. This will be the approach taken going forward.
Toolmakers wishing to use this library are assumed to be competent to work out which is the version of the library they wish to use, by consulting the changes list and reading versioning notes etc. As bugs are being discovered, and fixes are happening, all the time, we can't really say this update or that update is more correct than the last 
. All versions and source from: 
(link) (You can navigate from that page to the SVN and everything for the project)
Full change history.