"I'm a big fan of The Unarchiver - it will unpack just about anything and its FREE.
Right now I use 7-Zip a lot, since it has multi-threading support and can maximize my CoreDuo, but Unarchiver is great as a swiss army knife unpacker."
"Somehow 7-zip hasn't made it onto my system, but it is a great one.
What has made it on however is unrarX and Stuffit Expander. Stuffit isn't exactly great but it is required for some of the old legacy compression formats."
