For those with an Intel based iMac ies4osx will install Internet Explorer on OS X. You can choose which versions you install (or select all of them) then the program downloads and installs everything automatically in to the applications folder.
A rather suitable icon.
The program works by using Darwine to run windows based applications. Full installation instructions are available on the ies4osx website.
I found Internet Explorer to work without any problems and perfect for testing websites when building them.