Regarding Jeff's list of big APIs: I am pretty sure that common does not mean "Gold Standard".
No need to keep a manual list of widespread API. To get a list check out http://www.programmableweb.com/apis/directory/1?sort=mashups .
Since I like REST as loose standard, I'd say that an API is "Golden" when it makes sense and is intuitive.
- Twitter ( http://apiwiki.twitter.com/ ),
- Github ( http://develop.github.com/ ) and
- Last.FM ( http://www.last.fm/api )
… all make the most sense for me and are well thought out (as Brian already pointed out).
In my current daily work I also work a lot with OpenSocial, where URIs feel very natural but also extend the REST standard in its own way.
If you like it strict and safe, use SOAP.