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 ...
StuffIt's SITX has algorithms for compressing uncompressed audio formats built into their format, and I've known a lot of people who use it when Digidelivery/Aspera isn't an option and we have to resort to FTP et al.
Luckily they are now Win and Mac (used to be Mac only), so it's become a universal format, although not quite as deeply-compatible as say ZIP, ...