Shout (GitHub: erming / shout, License: MIT, npm: shout) by Mattias Erming is a web-based IRC client. The UI is very impressive, it feels lightweight and fast, but still familiar to die-hard IRC users like me.
I still use an IRC client on a server with tmux, and I probably always will, but there are times when I get forced into using web-based chat services. This is usually for work, and I pretty much never like them. The thing that's good about Shout is you can run it on your own server, so you could install an IRC daemon on a server somewhere then set it up to connect to it automatically.
This means your colleagues that hate console software can use a friendly web interface, while you can hang out in the shell like a civilized person. There are commercial services that offer IRC backends and friendly web frontends (Gitter and Grove are good examples), but you may like to host your own or hack Shout in some way.
Shout is built using a simple server based on Connect and Socket.IO that maps the client UI to a real IRC server. It has a Grunt build script, and treats IRC commands like plugins so it should be easy to add new ones.
The client-side code uses Handlebars for the templates, and the CSS looks easy to modify. There's a folder called themes but the example CSS file is currently empty, so I'm not sure how the authors intend that to be used.
The underlying IRC client implementation is slate-irc, which I seem to remember TJ Holowaychuk wrote for a cool Node desktop app. Mattias Erming is now a maintainer, and he's been committing work to the slate-irc project on GitHub.
Overall Shout looks like it has a lot of potential, and I've been impressed by what I've seen so far.