Parallels and VMWare have always been pretty good ways to get the job done. The bad thing about them though is that you have to install and boot an entire OS for just 1 application.

A much better solution is ie4osx. This is a very very nifty solution by Mike Kronenberg that allows you to run multiple versions of IE as standalones on an intel based mac using Darwine. A couple of huge benefits of ie4osx over the usual emulators:

• Less memory usage – Since you don’t have to run an entire OS, you need a lot less resources.
• Faster workflow – Obviously instead of having to boot an entire OS and make several commands within your emulator, it’s a lot more fun to just startup IE like any other application.
• Conditional Comments work like a charm – This is nothing Parallels or VMWare can help, but more a problem on Windows itself. You can run multiple versions of IE, but testing Conditional Comments is difficult. All versions will identify themselves as the highest version installed. Since Conditional Comments are usually one of the main reasons you are testing anyways, this is a very good thing.
• Easier to test through a local webserver - Since it runs locally on OS X, IE will use whatever hostname configuration you’ve setup on OS X. This means that if you use the webserver capabilities on OS X as a local development environment, you can access your websites through the hostnames you use in your other browsers. This can’t be done as easily on Parallels or VMWare.

Don’t expect it to work as snappy as your other OS X apps, but if you’re a developer that wants to fit IE testing smoothly into his workflow, ie4osx is exactly what you need.