I would echo Freesound.org and Sound Bible as great sources of sound effects.
As an aside, I know some of the guys who put their stuff up as Creative Commons on freesound have expressed disappointment in their forums that a lot of youtubers neglect to credit them. It would be cool if our community could keep their community happy by acknowledging their awesomeness when its a term of use (and sometimes even when it isn't!). In theory it should be a great mutual backscratching - they provide great raw materials for youtuber's art which in turn provides their sound engineering with exposure too. The description box is a great place to do this, if it doesn't work in the video itself.
(Sorry, that probably came off way more preachy than intended and is definitely not directed at anybody specifically. I just think the Freesound guys are pretty awesome...)