Music and Sounds
A lot of free use music. If you've got something specific in mind, it can be a little tricky to find something that matches, but there are a lot of options.
Nicely organized, so it's a bit easier to find what you want.
Free Music Archive. The 'Browse by Genre' option leads to some pretty interesting things.
Not as easy to search as the previous two, but there are some very unique tracks here.
Another nice music database, with some fun sections to explore (radio drama is a personal favorite)
They want to sell you a product, but there are a good number of free downloadable sound effects here.
Another good collection of free sounds. They also have a pay section, but I haven't explored it.
This is actually a site for flash developers, but they have a lot of sound effects for download.