The Online Etymology Dictionary:
“I began this project after I looked one day for a dictionary of word origins online and found that there was none. There were dictionaries with definitions, some lists of slang words and their sources, and some sites that listed a few dozen of the strangest etymologies of English words. But there was no comprehensive list of the words we use every day — words like the and day — that told what they used to be before we got them.”
Also: Word & Phrase Origins (concerned mainly with the ‘stranger’ etymologies).