iMovie Audio Clips
If your doing a lot of YouTube Video Logs, there are many sites that offer great background sounds. You should know two of them that have excellent libraries for great background music aned effets when you need it. There's only so many times that you can use the Apple's Playful Sound Effect. Why not be unique and invest in something that will spice up your videos.
Pond5 - This is an open marketplace for buyers and sellers of royalty-free clips in all categories. Some of the clips can be pretty expensive. $34 for a 2 minute background music? Just make sure you know what your buying. After Effect text and Pre-Key Footage are really cool and really can add some professional touches to your videos.
Stereobot - Lots and Lots of background sounds. They can also be pricey but have a lot of good Playful alternatives to spice up your videos.
Audioblocks - A subscription service where you can download unlimited amounts of audio clips. Great selection and mix of music types.