Came back from the Realcomm2008 conference last night. During my time there, I came across a pretty cool and useful new online real estate site called eRealInvestor. It happens that I met the Founder and CEO, Mike Genstil, on a ski trip with mutual friends a couple years ago.
Anyway, his company provides a good suite of analysis tool for residential and commercial real estate properties. There are 14 real-time financial metrics that you can adjust on sliders (i.e. cash flow, cap rate, gross rent multiplier). You can also conduct a side-by-side analysis of up to 3 properties. They are already integrated with LoopNet and The Wall Street Journal's real estate site.
If you don't already know about LoopNet, it's what I think is the best commercial real estate site out there if you're looking for properties to rent or buy. I used it when my old company was looking for office space. They have over 950,000 unique visitors per month, 2.75 million members, and over 595,000 listings.