Linux users might know Wine the windows wrapper that allows them to run Windows applications, since wine is multiplattform you can use it as well on Mac OS X. But it means a lot of config file editing, try & error and some luck to get good working results.
Here WineBottler comes in, it allows you to transform Windows applications to Mac applications using a simple interface.
Installation is quite simple just drag'n'drop Wine and WineBottler to your application folder.
After starting WineBottler you can either install a predefined prefix which are some standard apps configurations built in WineBottler or create a new one by creating a custom prefix.
As a web developer i need to check my webpages and applications in Internet Explorer to make sure everything is ok, so i choose Internet Explorer 7 from the predefined prefixes.
Just click install to start.
Few seconds later i have a Internet Explorer 7 application ready to start and share.
It's not 100% perfect but good enough to test websites.
WineBottler is free and open source. It's using wine as backend so you can check the WineDB for compatible windows applications.