What online resources are available to trace back the origins of words?
Ideally, this would be a searchable web dictionary like Wiktionary but listing the etymological chain along with some sort of proof (or a weaker reason to believe) - links to descriptions of the relevant sound change patterns, or to old texts where the word was used, or to that paper in which some respected linguist speculated about such-and-such a change having happened n centuries ago.
I'm particularly interested in Slavic languages and their relationships to other branches of PIE, but I'd be curious about how others handle this problem too, so please consider it a general question.