sims3tools provides a set of tools for the Sims3 modder, based around the s3pi library. It includes:
a package editor (s3pe), which allows most useful resources to be edited;
an Object / CAS Part cloner (s3oc) for creating a base for modifications;
a sims3pack packer/unpacker (s3su), which simplifies handling the content of these files;
a string table editor (s3se) for allowing string table resources to be efficiently edited;
plus a number of additional utilities.
QA releases have the latest bug fixes. Public releases have generally proved stable for several weeks!