Trying to find which supplier has that just-released new widget in stock can be very time consuming.
Fortunately, there are some search engines designed specifically to check many suppliers all at once, and to give you a quick overview of who has what.
If you know of a search engine that should be included here, please let me know via private message to rcurl. All comments are welcome.
For now I will limit this listing to search engines that return both price and
availability. There are loads of search engines out there that will tell you if a part is available but require you to issue an RFQ in order to get pricing.
My experience has been that all of these sites will sometimes miss reporting a part that is in stock. It's a good idea to check more than one of them if your search comes up empty or if prices are too high.
No registration required. U.S.-based suppliers only. This site searches over 20 suppliers. I've had good luck with it, although if your search query contains more than one term, you may get erroneous results. It's very basic, also very handy.
No registration required. U.S.-based suppliers only. In addition to seraching by part number it will allow you to refine your search by manufacturer, supplier, lead-free status, packaging, ROHS status, case/package type, mounting type (surface or through hole), and lifecycle status (active, end of life, obsolete). This is one of the few sites that allows a search by component category. They also provide free project management tools.
Requires registration. Free for qualified OEM's & CEM's. This site searches suppliers in a multitude of countries. While they don't show you prices at a glance, they do give you a link to pricing for each supplier that has your part in stock. Responses are grouped by "The Americas", Europe", and "Asia". In addition to showing items in stock, it will find "brokered" inventory. This inventory is usually not actually in stock, but can usually be acquired and shipped in one or two days. Netcomponents provides a Datasheet Locator
and a currency converter
You can use this search engine in two modes. You don't have to register in order to do a quick search, but if you do register you can blast out RFQ's to a number of suppliers all at once. They search about 35 sources.