There are many places for professional engineers, students and hobbyists to go online to find information on electronic components. Here is a useful guide and quick list to some of the more popular places to get datasheet, specs and other information needed to design a new piece of electronics.
The sites on the Internet are broken down into these categories.
Original Source. Edit
These sites are the authoritative sources for a component. This is the best place to get a datasheet or a spec on a current component. There are very few datasheet search sites that allow you to search across many manufacturers. Most of these sites are the manufacturer so you must visit each site to search for thier parts.
SupplyFrame datasheet (www.supplyframe.com): one of the only search engines that support many manufacturers and pull the datasheet and spec data directly from the manufacturer's site. SupplyFrame unique gives you links directly into the product folders of the manufacturer site for additional information. They also give you inventory levels and links to distributors where you can buy the part.
After that, you need to go to each manufacture individually and search for the spec you need. Sometime Google can help you but often you have to figure out the site of each manufacturer.
Scraper Sites Edit
These sites don't ask permission to use other peoples data, they take it anyway they can. Mostly they just copy it off manufacturer sites. Most of these sites are outside the US so apparently there is little to stop them from selling off other company's copyright information.
AllDatasheet: This site only covers datasheets. The site is located in Korea and you can see right away that they have language challenges. They do have a lot of datasheets.
DatasheetBank.com : This is an excellent site. Datasheet Search Engine and Free Datasheet Download.
DatasheetCatalog: This site is in Romania and also only covers datasheets. It's somehow related to another site Ortodoximsthat is in an unfamiliar foreign language (Romanian perhaps) and has religious symbols on it.
DatasheetArchive: This site is in Europe and focuses on obsolete components. If you are working on something that is old, then this is an excellent site.
Blatant Marketing Sites Edit
There is no free lunch at these site. They require you to register to access their information and you must agree that they can sell your information to other people. If you are lonely and like sales people to call you, then you could try these out.
ICMaster: This site is part of Hearst publishing. They buy their data from another company overseas that gets the data using whatever means is necessary. Content is pretty good but limited.
GlobalSpec: The data on this site is spotty. They only include data if the manufacturer paid them to put it on their site. If the data is there, it's pretty good. Be prepared to have barrage of email or print catalogs land on your desk if you use this site.