Online design tools are getting more and more popular. I've gathered a few of the more popular ones. I haven't tried any very seriously as I do most of my desigs (if you can call that designing) by hand. If you've tried any of them, please let me know in the comments what you think.
FROONT runs in your browser and allows you to see what you're designing in the same way users will. Present comps online on all devices. Instead of giving static images to your developer, provide them with fully functional HTML and CSS code.
Jetstrap is a 100% web-based interface building tool for Twitter Bootstrap. No software to download, just log in and build. Your work is accessible from anywhere.
Designing in the browser is the only way to ensure pixel perfect execution. Use modern tools like web fonts and CSS3 but without the interruption of shifting back and forth between your text editor and your browser.
We love our frameworks and tools just as much as you love yours. Divshot lets you build visually with most popular front-end frameworks including Bootstrap, Foundation, and Ratchet.
Webflow automatically generates beautiful code for you as you design. You don't have to write code yourself or hire a developer to do it for you.
Use Drag and Drop and WordPress to create 100% pro, mobile-ready sites without coding.
Finally, this one is not an online app, but it had a very successful KickStarter and the buzz around it is very positive. They're even having a preorder sale right now if you're interested. I'm not affiliated with them nor I have tried it, but it looks solid.