It would be nice if there was some central repository
Ummm...
and
http://www.arduino.cc/playground/
take you to a whole bunch of good stuff. Not EXACTLY the database of code for specific sensors the poster suggested, I know.
And here's a useful trick:
Put the following search into Google....
site:http://www.arduino.cc/ sht15
... and you'll get 6 references to the sht15 (humidity sensor), as you would expect... but only references found on the Arduino webpages, so you have some indication of their applicability to Arduino work.