Winrar is for Windows. UnRarX is the mac alternative. Unrarx for Mac is free and is compatible with Both Tiger & Leopard OS X. Mac OS X doesn't come with the capability to unrar a file. I doubt if stuffit expander can do it. Now, I can accept all the rar files that I get from my windows friends. As the website describes:

UnRarX is a Mac OS X Cocoa application that allows you to expand rar archives and restore corrupted or missing archives using par2.