Wine’s pretty good these days. On its own it can install and run programs like Microsoft Office under Linux. This is thanks, in part, to contributions made to the Wine project from the Codeweavers gang.
Their flagship product, Crossover, is a commercial product based on Wine which lets you install many products from the Microsoft platform.
Update: You don’t need to register your email, just download the version that you want.
Oh, and there’s an OS X version too.