I like Soundsnap a lot. Took me awhile to find it months ago, because it was buried under many other spammy and commercial links when I searched for "free sounds". (It's since risen to #3 on Google.) This left me muttering under my breath, where's the audio equivalent to Flickr? (I know Zoooomr has audio annotations, but since the focus is still mainly on visuals, it's not a contender.) Technology developments related to sound usually lag behind sight — witness all the buzzstorm about faster GPUs, with not quite the same attention paid or comparative progress achieved with the latest sound cards. But nevertheless, that's left some pretty wide opportunities on the market…
I'm not referring to pay-for sound sites either, because while there may be legitimate businesses of great use to professionals like prosoundeffects.com, many can't argue that the simplicity of a Creative Commons license and ease-of-navigation makes it super-fun and low-hassle to get sounds for use in a home production, be it a DVD for the grandparents or a short film to share with friends.
Which brings us back to Soundsnap — years ago, I used FindSounds. It's still around, but hasn't changed a lot. Many of the sounds it finds are low-quality, and it still has unsuitable defaults for this day and age: 8-bit and 8000 Hz? Come on! Not to be too harsh; I'm thankful for it being available, as I am The Freesound Project and others, but I haven't found something yet which surpasses Soundsnap's ease-of-use. Its embedded previews (both a waveform and an audio player) are fantastic. The download links aren't hard to find. And categorization is pretty cool, offering both a breadcrumb-style categorization and tags. It's got both a wealth (1000s!) of incidental SFX and music samples to choose from.
Eventually, I figure an esteemed publication, a sonic counterpart to Smashing Magazine, will do a compilation of all the great sound search sites. The closest I can think of right now may be Peter Kirn's brilliant Create Digital Music. I'm looking forward to it, or I may have to do it myself. 
Just wondering……. do you have like a 40 hour day or do you just do things so fast relative time appears slower?
Torley runs on Torley time lol. Creative people always run in a different dimension anyway which is why mundanes always make problems for them.
Nice find Torley! I love using sites like this and to counter people that swear up and down that all machinma with music must be violating some artist's copyright.
Thats supposed to be "like this and mobygratis" but obviously i typo'd. apologies.
Whilst soundsnap is definitely the easiest to use site, the freesound project is awesome, and has a huge array of sound effects :
http://freesound.iua.upf.edu/
also for one off commercially produced and recorded sound effects from libraries I use http://www.sounddogs.com/
@Ewan: The latter. A lot of the latter.
@Ann: I'm a big Moby fan. Haven't seen that before, thanks for the tip! He continues to have a distinguished voice in an ocean of nonsensicalness.
@Dizzy: Some of these sites with a lot of gems, I wish they were easier to search/organize! Looks like the market is wide open for a better sound meta-search that really catches on.
Soundsnap is certainly the place to which I go first. Apart from the convenience of the interface, it has a simple and easily understandable licencing system: "it's all free, use it how you like".
Soundsnap also absolutely excels in providing bleeps, buzzes, twings and electronic noises in general.
I do use Freesound, and have indeed contributed to it, but I confess that I do not think I properly understand the ramifications of the Sampling Plus licence or whatever it is called, and this worries me. Still - it is better for detailed source recordings I would say, particularly those of an obscure mechanical nature, and the quality is higher.
SoundDogs is very much aimed at the commercial market, and while it has a wide selection of stuff, the costs of the proper quality files are prohibitive for the average SL developer. I would rather obtain something _close_ to the target for free, and edit and blend it with Audacity or some such. I could perhaps see myself using it for a very specific topic where the sound had to be precisely accurate - perhaps a replica machine of some sort.
I read your post the other day and just recently had a need to find a sound for a client. I checked out soundsnap and found a sound to start off with. I love the licensing here. Optional/Recommended attribution, and free to use however you like as long as you don't hand out the original sound commercially on its own, in a collection of sounds, or without transformation. I ended up altering the pitch of a bicycle bell to be lower so that it resembled a trolley sound. Awesome!
@Ordinal: I like the initial simplicity of Creative Commons; it's definitely appealing to those who want to take action and use media without worrying about the consequences of misunderstanding.
I used to buy some stock sound sample CDs, but found them kind of wasteful after I chose only to use a few clips out of the 100s (or 1000s) on a disc. I like paying for what I actually use. But, in that "grab bag" dept., I haven't found a better deal than this: http://www.masterbits.de/mp1_e.htm It's mainly for music production but has lots of weird sounds, including one-shot percussives, that can be used elsewhere. It's somewhat dated now and hasn't been updated in a stretch of years.
@Dedric: Nice audio manipulation! I sometimes think, "What would Ben Burtt do?" and he's had so many remarkable insights in sound design.