Once...probably never again unless I start the company
The big problem I have with "working for equity" is the offer is generally made by someone who doesn't understand / appreciate the value of technical talent vs. business talent in the early stages of a startup. Now - if the person offering it is either another developer (getting their hands dirty) or a boots-on-the-street sales guy who has a proven ability to close deals (and is willing to work on the same basis), they might be able to talk me into it. But a marketing or "strategic" guy? Never...been there, done that, got the t-shirt...it wasn't pretty...