here's another one:
+ Easy to navigate thematic organization
+ Curated generally useful incidental sounds
+ Free commercial use, provided not main focus (full license)
- relatively small collection (a couple of thousand sounds in my estimation)
This is a Community Wiki answer, feel free to add your finds...
Field Recording Databases
Freesound >> http://freesound.org/ (added by yunque)
+ Search by license, format, bitrate, sample rate, tags, etc.
+ Open project, user-based (can search similar sounds recorded by the same person).
- Variable quality.
British Library ...