Opening Up to Pure Geek Goodness
The real power of open APIs is not so much the particular, obvious things they let you do, but that - as with all open endeavours - they remove unnatural obstacles so that the only limits are your ingenuity.
For example, an outfit called TruePath Technologies has plugged network monitoring into Google's open Calendar API to create something no sane - or uninspired - individual would ever have dreamt of. (Via Digg)