Douglas Harper, creator of Etymonline, considers himself an amateur linguist and warns
... if you're a professional linguist or a serious student of linguistics, you shouldn't be doing your homework here. This is for the rest of us.
Nonetheless, it is a popular web resource.
Other sites (Wiktionary, dictionary.com, wordnik) seem to focus on definitions at the expense of sense evolution. If you want more etymological details (context of word usage) online, where can you go, short of buying the sources used to make Etymonline?