ies4osx is a convenient way to run Internet Explorer natively on Mac OS X, without having to pull up Parallels or reboot into Windows with Boot Camp. Lifehacker (which is now one of my favorite sites) posted it this morning at Run Internet Explorer on Your Mac with ies4osx.
I haven't used this package on the Mac, but it's derived from the latest version of the IEs4Linux package which lets you run Internet Explorer natively on Linux. I've used IEs4Linux on Ubuntu Linux 6.10 and 7.04 and it really works rather well.
It's particularly handy for accessing financial and corporate sites that require IE for their functionality. That's less of a problem now than it was 2 or 3 years ago, thanks to the continued growing popularity of Firefox and Safari.