Yes Yahoo is a latecomer here but you have to appreciate the effort to expose not only web search, but also local, news, images and video. For now as per the FAQ they offer a REST interface and will implement SOAP only if there's significant demand for it. Access is free but capped (by IP address, now that's a bit strange [*]). There are a couple applications so far. Here are the obligatory weblog, mailing lists and wiki, some support from O'Reilly, and of course Jeremy Zawodny is part of it.

As usual, it's non-commercial by default unless you ask and obtain permission. The SDK provides Perl, PHP, Python, Java and JavaScript code samples - not a surprise coming from Yahoo.

[*] Update: Phil clarifies the IP quota for me. Good point, I had read too quickly without realizing this lets your application be accessed by more users without hassling them to sign up for a developer key (which of course doesn't make any sense). Yep, Yahoo has the upper hand now.