1) .sit files. You can go to all the trouble of fixing creator/file type, or just drag and drop to the icon of StuffIt Expander. It doesn't matter if it is a "Limewire document". If Expander can't unpack the file, then trash it.

2) for mp3's: get "iTunes Associator" at versiontracker.com. Freeware.

End of story.