As with the music category, finding true repositories of full public domain sound effects is difficult. The only category here is "Mixed Licensing.
- Orange Free Sounds
This is my favorite on the list. The licenses are all Creative Commons. Some disallow commercial use, some don't, but all of them require attribution. There are sound effects here for nearly any purpose.
- Sound Bible
Sound Bible has a wider variety than Orange, but the navigation and search are not as good, and it's less organized. The sounds come from various creators, so the licenses are varied. Some are public domain, some are very restrictive.
- Free Sound Effects
This site is mostly paid sound effects, despite the name. However, the free sound effects that are available are good ones. This is in the "Mixed" category because the majority of these sounds are paid. The free sounds have a very unrestrictive license, but it isn't a typical Creative Commons license so be sure you read the terms.
All the sounds h ere are licensed under Creative Commons, from CC0 public domain licenses, to more restrictive ones. There are some great sounds here, as well as some pretty bad ones, so finding something great can be harder than others on this list.