Craig,
As for RAR, ZIP, etc. utilities, I couldn't help you out there since I do it all from the terminal. OSX has built in support for extracting ZIPs, TARs, and a bunch of other formats, but I don't know about RAR.
For AC3 and other video playback stuff, I recommend Perian:
Perian - The swiss-army knife of QuickTime® components
Some other cool things are Adium (for a messenger, but it might not provide what you need business-wise):
Adium - Download
Growl is nice, and it provides pop up notifications for various things:
Welcome to Growl!
Also, since you are using a laptop, be careful about the things you have running in the dashboard. They are
always active and running. This means they can drain CPU usage and lessen your battery life. The average RSS thing is fine, but all that flashy stuff is a bad idea.
If you need any custom RSS feed things for the dashboard, let me know. I made that
StrafeRight RSS Widget awhile ago.