Pegedit is a web-based development environment for designing PEG grammars (yes, I know that's redundant). It combines Ace source code editors for input with PEG.js to generate the parsers and D3.js to help visualize your parse trees.
Visit the website at http://pegedit.cspotrun.org. While I'll do my best to keep the site updated, I don't have any kind of clever build system that deploys my commits, so this repository will always be the place to go for the latest, most authoritative versions.
The wiki has some details on building this site from source and should soon contain some decent user information. While the site did have a nice Joyride walk-through, the UI has changed so much as to make the tips no longer fit.