Reader pl1ght added to the comments section on the article about Power Archiver a shout to take a look at 7zip for Windows. And after the final test, we have to say 7zip gets the win, all the way. It's not pretty or ugly. It's simple and fast. Best of all, it's Free and open, supports all the typical Windows and *NIX formats. For those concerned with it, it features better compression ratios than anything else, not just in the 7z format but even in plain old zip. In others words. WinRAR, you have been displaced. Finally, no more nag.
7zip is mostly licensed under LGPL, except for the unrar code, which is in more restrictive terms, as dictated by the rar developer. We prefer to take the open source solution here if there is one, so this is a big plus. Not just out of software politics (though we care about that) but also, for purely practical reasons. GPL stuff often produces the best code, and if its got more than a trivial user base, it's never orphaned.
Hosted on sourceforge, of course. Get it here.