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Ok, it's not hard to guess what this does! Dictionary.com lists definitions, etymologies and thesaurus entries from many popular dictionaries.
How Stuff Works
How Stuff Works is a great technical resource. They cover lots of technical topics. Their emphasis is on electronics and computers, but there if you want to know how it works, chances are: they know.
This on-line encyclopaedia is one of its kind. It's mission is to become the "sum of all human knowledge". To this end, anyone in the world (even you!) can go and add and change information in the encyclopaedia. It might sound suspect, but wait till you see what's available!