If you own a Mac and you’ve got a variety of video formats you need played, you could just go ahead and downlaod the most excellent VLC Media Player. It’s certainly a heavy duty player with ripping and encoding abilities to boot.  However, if you’re just looking for a way to play your mountain of 30 Rock episodes  that are all in AVI format, you can have Quicktime play these by downloading and installing the Perian, the self professed swiss-army knife for Quicktime.  It’s a preferences pane add-on that allows Quicktime to play a variety of different video formats.  It’s a quick download and an even quicker install and after checking it out I can’t believe this wasn’t one of the first things I installed on my Mac.  You can download it here.