Virtualbox works well enough, and is free. You can run it in seamless mode, which makes the windows program look like it's running directly on your Mac desktop. I've run Virtualbox on my Mac, and on a Linux box at work for some time.

Parallels or VMware Fusion are commercial options that may have a little more functionality, but obviously cost money.

In some cases though, you can get away with running the windows programs through CrossOver, which is a supported implementation of WINE / Darwine.