Octopart is an invaluable tool for me when I'm doing design work or documentation. It is a Google for electronic parts. I can use it both for directly searching part numbers, or use its hierarchical search functions to find the part I need for a particular purpose.
It turns out their existence and survival was a very close run thing. They failed to close a critical round of financing during the financial crisis, and very nearly went out of business. Michael Arrington tells their story in Techcrunch.