BBC has launched a database entitled BBC Sound Effects which, at the moment, contains 16,016 sound effects freely available to download. You can browse through the thousands of sound effects here. All files are available for use under the RemArc license, which you can view here.
All files come as WAV files and range from "A large Circular saw cutting 12-foot planks" to "Restaurants: Large Canteen, quiet, loud hiss of steam". The site also comes with a convenient search function,