As the designer, you can set up multiple projects, specify clients, and upload images (in JPG, GIF, or PNG format) for review. Your clients log in and add notes by selecting a region of the image and typing in a text box. When you later check out their feedback, you can see comments linked to specific regions all over your image. When all parties are happy, they can "sign off" on your project and you can get paid.
You can create a free account at Getsignoff that delivers an admittedly meager upload limit of 10MB -- I used 25% of that limit with my first project -- and just one client. The Pro version gives you a generous 10GB limit for $30/month and unlimited clients. The service is pretty useful, but the free account is clearly designed to reel you into a subscription -- which seems a little expensive for my taste. If there were an intermediate $10/month option, I'd be all over this. What about you? [via Lockergnome]