Red Pen
… lets you upload your design, share a short URL, and get live, annotated feedback super-fast. It remembers you— there's no logging in, no project management, no bullshit.
iPlotz
… wireframing, mockups and prototyping for websites and applications
Mockingbird
Wireframes on the fly … an online tool that makes it easy for you to create, link together, preview, and share mockups of your website or application.