I can’t remember how I stumbled across this but I can’t believe it took this long (either for this to exist or for me to find it). The Unarchiver is a replacement for the built-in archive unpacker program in Mac OS X.
Crucially it can handle .rar and stuffit (.sit) files (and a lot more). This means one unpacker to rule them all, no more UnRarX and StuffIt Deluxe.