Currently, the best place on the internet to find out the latest and greatest news for Interactive Fiction (or I-F), is at the newsgroup rec.arts.int-fiction. The group has an almost-monthly FAQ. The group also uses the if-archive to store free (or shareware) I-F related materials (located at ftp.ifarchive.org).
Good question. The answer is simple, though. SourceForge provides source-control access to files, which allows for easy versioning of source code. Also, it provides a good bug/feature tracker, as well as mailing lists and a binary release archive.
Simply put, this is the only IF language I use. If this project takes off, we might support other languages, too.
Contact the project admin to get yourself registered as a developer. You may need to have a new module created in the CVS hierarchy.
The de facto standard is Graham Nelson's Inform Compiler. Other compilers include the Glulx compiler and the Zasm z-machine assembler. I'm sure there are plenty of other compilers out there.
This is a list from my last search (not including the Inform-related issues above).