Debuggex is a very well done and useful site. It reminds me of http://www.colorzilla.com/gradient-editor/ in terms of utility and slickness of design. Alex Sirota of that site uses a sponsored area that is not obtrusive.
It would be interesting to do a SaaS play and charge on a monthly basis, with all updates available to all users. And then allow some (very basic AND time-limited) use by free users, only for trial purposes (not freemium).
You've got a really targeted and engaged base of returning users. You could add a text link to a product or service for hackers with an affiliate link, you'd probably get pretty high conversions with this audience.
