jomaxx - 7:14 pm on Mar 11, 2007 (gmt 0)

High pagerank, high Google rankings, high traffic as measured by Alexa, strong brand recognition. None of these are prefect by any means, but they're okay indicators for how popular a site is. Popularity doesn't necessarily imply profitability or even business acumen of course, but on balance it's a good place to start and there's a lot of other lessons to be learned from top sites.

You can also look at frequent high CPC bidders in your field. If they can really make 5c or 50c or whatever per click, then they likely have a sound business model.