Ted's answer is only partially correct.
The last Mac version of IE was 5.2 and has not been updated in 6-8 years. It was never ported for Intel Mac so it will NOT run under OS 10.7 or 10.8. It may run on OS 10.6 but, even on a supported processor, it is incapable of properly rendering most modern web sites.
Did you find a site that only works with IE? You can set Safari Preferences to show the "Develop" menu in your Safari menubar. That gives some "User Agent" choices that help you make web sites think you are running something other than Safari. You can also use alternate browsers like FireFox, Chrome, Opera, etc.
The only way to run a current version of IE on a Mac is to install Windows either through Boot Camp or a virtualization product that allow Win emulation on a Mac.