Finding the right parts can be a pain, especially when you’re new to electronics (as many Arduino users are). That’s why Octopart is so great: it allows you to search for parts across multiple distributors, telling you the price and quantity available from each one. For example, you can use it to find a replacement ATmega168 for your Arduino board, or a L293D to put in your motor controller shield. You can even find Arduino boards on Octopart, since they index Sparkfun. If you’re not sure what the number is for a part, but you know what it needs to do, you can narrow your search in the parts categories (kind of like how the Yahoo! homepage looked back in the old days, but just for electronic parts).
We’re especially fond of Octopart because it was started by a couple of guys who dropped out of grad school to do it. Plus, their third employee is a friend of mine from high school (don’t worry, though, I’d recommend them anyway). The guys are working like mad to improve the site and add features, so I’m sure they’d love to hear from you if you have any suggestions (firstname.lastname@example.org).
In conclusion, if you’re in need of any electronic parts, try Octopart.