SourceMogul has been a HUGE time saver when trying to source products for re-sale. I am able to quickly search many different websites to find items that meet my criteria for reselling based on ROI, sales rank, $$$ profit, etc. It even tells me if I'm restricted from selling a certain product on Amazon.
It helped me find many products I wouldn't have known about otherwise. I've only been using it a few weeks, but I've already made a few hundred extra dollars profit and I'm sure there will be lots more to come.
And in comparison to some other arbitrage sites I've seen, this one runs much more quickly and the site is laid out logically (and looks pretty). I'm able to see all the data I need right away to make my purchasing decisions.