There are many others out there, but here are some of the slickest:

Interactive version from the NYT where you can enter and save your picks and compare your results to others over time.

ESPN’s is kind of nice as you can drag a pick straight through to the final and has popup background info:

The NCAA.com version has a bit of background info on each team in popups:

NBC’s bracket let’s you fill in and print out your picks, provides analysis on each matchup, and has prizes(?).