Yahoo, MSN and Google all provide powerful web search APIs so that developers can build custom applications. Google and MSN use web services to return their data while Yahoo returns its data via XML. I use the APIs often to retrieve back links and pages indexed data and have found that MSN and Google have always been relatively accurate (1-8% variance from actual). Yahoo's data for pages indexed has always been pretty accurate too, but I have found the back link count to be way off at times in the past. However, since the switch for "search commands" to Yahoo Site Explorer I am noticing better accuracy in the back link data. I can't say it's absolutely perfect (and probably never will be), but it's a big change compared to the numbers I was getting from Yahoo a few months ago. The variances I am noticing right now are in smaller, less authoritative sites.