Quick note, rename any .asf/.asx files to .wmv to get Windows Media Player to open them.
Most stuff can be viewed with MPlayer, including avi, mpeg, and wmv. Very few avi/mpeg need to be viewed in Quicktime only. About half of .wmv need to be viewed with Media Player. If you haven't downloaded the DIVX codec aleady, make sure you get it.
Mplayer:
http://mplayerosx.sourceforge.net/
As far as opening .sit files, that's weird. Stuffit Expander comes with OS X, but maybe you don't have a new enough version. It used to be free, but I can only find a trial version of it at
http://www.stuffit.com