What app opens .sit extentions?

stuffit expander

.SIT is the old Mac StuffIt Expander format. It’s fallen out of favor compared to Mac’s new native compress format’s like .zip and .dmg, but some files still use it.

@rainsmoker: Beat me to it.

This site is worth bookmarking

Stuffit makes it hard to download their expander, and their install process is itself annoying, so I highly recommend The Unarchiver, a free and simple download that will expand any file you need—including, of course, .sit files.