There's currently a poll up in another section of the forum where you are asked which is better, Yahoo! or Google. At the time I voted it was 21 to 0 in favor of Google. Alas, my vote too was for Google.
However, I do know one area where Yahoo! has the advantage in spades over Google and it's one that's very relevant to all the PBBG developers on this forum. The Yahoo! User Interface Library (YUI for short):
http://developer.yahoo.com/yui/It's a great collection of JavaScript and CSS designed to help you. If you're like me and you're not interested in learning JavaScript at all then you can use their pre-packaged, well-documented, complete with examples user interface widgets in your web pages to get auto-complete, tabs, trees, tables, etc. If you are JavaScript knowledgeable already you can use other code they provide to make your life easier.
In addition they offer a great set of CSS to help you remove browser differences in how your pages render on IE, Firefox, and Safari, lay out your pages quickly without going nuts, and handle font sizes in browser and machine neutral ways.
Seriously, this library is all about saving you time and offering you cool stuff ready to use. Yes, you can go get Prototype and Scriptaculous or one of half a dozen other libraries (like Google's own GWT) and get part of this or piece it together from a dozen different places, but this is one stop shopping and they don't skip on lots of extras (e.g. you can have your pages refer to the CSS on their servers from your web pages and you don't have to pay the bandwidth costs of shipping them to end users).
