The BBC has made 16,000 Sound Effects in WAV format free to download for use under the terms of the RemArc Licence. The Sound Effects are BBC copyright, but the RemArc Licence permits them to be used for personal, educational or research purposes.
The entire library is searchable and a database containing metadata about the Sound Effects is available on a dedicate site here.
Examples on the first few pages of the database include a petrol engine, a tropical forest, morse code being tapped out, announcements and the bell to start a wrestling match, a steam engine and footsteps.