Database with organic reaction mechanisms (curly arrows)
As far as free online resources go, I've found the following useful:


Organic Reactions (organic-chemistry.org)
Name-reaction.com
List of organic reactions (en.wikipedia.org)


I would also strongly recommend acquiring a copy of March's Advanced Organic Chemistry at some point. I think it's fair to say that it's widely regarded as one of the most essential reference works in the field of organic chemistry. It's concise, more comprehensive than one would've thought possible, beautifully organized, and contains thousands of primary literature references. While not every reaction is presented with an accompanying mechanism, many are, and useful exposition is typically provided.

Another book you might find useful is Strategic Applications of Named Reactions in Organic Synthesis. More than a mere reference text, it includes an overview describing important applications for the reactions it covers, examples of uses in practical syntheses, as well as comprehensive stepwise mechanisms.