Yahoo pump out loads of tech stuff all the time - YUI, Hadoop, YSlow, YUI Compressor are all hugely respected and popular tools. They've write articles and published research that are almost definitive references for things like web site optimization. They have a boat load of web service APIs that are used all over the place and some of them are very innovative. It's worth just browsing around http://developer.yahoo.com/ to see all the stuff they have going.
(No, I don't work for Yahoo or have any association.)
Yahoo is one of the largest collection of professional web developers, and they take that seriously. Of course, their money is in Average Joe offerings like News and Frontpage.
I do work for them, and for a developer offering team too boot, so this is quite biased, of course ;)