SimPE is a third-party program which allows players to view (and often work with) the "package" files that The Sims 2 uses. It is compatible with The Sims 2, as well as all of its expansion packs and The Sims Stories series. It's probably most used for viewing and editing - and occasionally fixing - various things about the Sims themselves, though it can also work with objects, clothes, and meshes. It can be used to edit Sims' names, but should not be used to edit a Sim's first name. More advanced features allow players to add memories to Sims, change the family trees, and resurrect Sims that can't normally be resurrected with the Resurrect-O-Nomitron or other ways in-game. It can be used to clone objects and to extract resources from a "package" file, which makes it useful for creators of custom content.
SimPE will associate the .package extension with itself; with SimPE installed, .package files will be tagged as "DBPF Package" files. This allows players to open a .package file in SimPE, but for players who have both The Sims 2 and The Sims 3 should note that both games use the .package extension, but use different file formats.
As SimPE is not compatible with The Sims 3, another version, called S3PE, was developed.