It is a Sims 3 package editor created by Peter L Jones originally just to demonstrate his s3pi (sims 3 package interface) library. The s3pi library is a gpl-licenced set of assemblies written in C# under .NET 3.5 and intended for use by any person wishing to make Sims 3 modding tools under a completely open source policy.
The tool sets out to be as "tight and light" as possible, so as to handle large files easily. Its resource editing capability is implemented either by opening external programs, with the data being streamed to them automatically, or by the user exporting files to be worked on followed by repackaging them.
You can learn about, and get support for, the latest versions of both s3pe and s3pi at our support forum on Simlogical or, in the event of the demise of that forum, you can obtain compiled releases and source code from s3pe project on SourceForge and s3pi project on SourceForge