Don't forget tracking numbers...
http://www.google.com/search?ie=UTF-8&oe=UTF-8&sourceid=gd&q=1z5xx8810344800919&hl=en&rls=GGLD,GGLD:2006-34,GGLD:en&wxob=0

Brandon

and USPS: http://www.google.com/search?hl=en&q=9101785091401023989447

Area codes: http://www.google.com/search?hl=en&lr=&q=area+code+650&btnG=Search

Also one for tracking Fedex and DHL packages. I don't have a tracking number to show the onebox though.

Temperature conversion as well (though I think that's a calculator function)

http://www.google.com/search?q=80+f+in+c

Google News Archive
Festival onebox http://www.eryi.org/SearchEngines/Twelve-Animals-in-Google.html

Hmm, they seem to have removed the German train onebox:

http://blogoscoped.com/archive/2004_12_13_index.html#110293196775959962
http://www.google.de/search?hl=de&q=stuttgart+trier&btnG=Suche&meta=

Chinese Festivals
http://www.google.com/search?hl=zh-TW&q=%E6%98%A5%E7%AF%80 (Chinese New Year)
Solar Terms
http://www.google.com/search?hl=zh-TW&q=%E7%AB%8B%E6%98%A5 (Lichun)

I posted an example a while back at http://www.heapseo.com/blog/2006/11/23/would-you-like-onebox-with-your-search-results-sir/ which showed a search for 'new york to san francisco' returning all of the news, local and flight info one boxes in the same result. Doesn't seem to do it anymore, guess it was jsut another UI test.

You forgot about reverse phone number lookup for the US.

What about Froogle results? Or is that considered morpher into Google Base?

The French Train OneBox still works:

http://www.google.fr/search?hl=fr&q=paris+marseille&btnG=Rechercher&meta=

via...
http://www.seoprinciple.com/find-a-flight-easily-with-google/15/