Trigger.io released their cross-browser extension builder as an open-source project[1] a bit ago. And we've been using it from before it was open source, and we love it. Saves a ton of time, highly recommended.

[1] https://github.com/trigger-corp/browser-extensions

Chromium installs Greasemonkey userscripts as normal extensions (minus update support). This tool turns them into XPIs: https://arantius.com/misc/greasemonkey/script-compiler.php

Hopefully, Navigator Extensions, NEX, will take off. http://my.opera.com/ODIN/blog/2013/07/30/introducing-nex

Genuinely curious: when you want a "cross-browser extension", why wouldn't a web-app do? Isn't an extension specifically something that requires non-standard browser APIs and hence can't be made part of a website?

some extensions allow you to hook into browser chrome.

while the actual work of the extension may be done in a web app, extensions get more access to the interface of a browser than a normal web app

example: the Buffer extension could probably just be a button that simply loads a JS file from their servers that does all the real work. WIthout being an extension, you wouldn't have the convenience of having the button on every page you view.

Extensions can augment other websites' functionality, e.g. using "Missing E" for making Tumblr suck less.