Just in case you missed it, we have an "Explore GitHub" page that should get you started with some repos that you may not have heard of before:

https://github.com/explore

The explore page offers a great snapshot. The search page is totally awesome too (seriously, it works really well) but only when you are looking for something in particular. It feels like there is a mind-boggling amount of potentially useful things sitting there that no one ever knows to play with because the people behind the projects aren't very good at PR or aren't looking for high usage of their product. I guess the main barrier to testing out projects/apps is the difficulty of installation. You have to know you want the product a lot to go through with the effort to get it up and going. I guess that's why recommendations can be a good guide to making sure it's worth the effort.

Not sure how obscure these would be here (I first learned about them on Hacker News), but I've found Backbone.js and Underscore.js to be extraordinary projects.

Links:

http://documentcloud.github.com/backbone/

http://documentcloud.github.com/underscore/

For a bit more information -- Backbone.js and Underscore.js are open source components of DocumentCloud, funded by the Knight Foundation's News Challenge, and are used to build this:

http://cl.ly/3N0Z

... and this:

http://documents.latimes.com/la-county-plastic-bag-ban/

http://thechangelog.com/ is a great blog to follow for interesting GitHub projects, it also powers parts of the "Explore GitHub" page that @mojombo mentioned.

I've really enjoyed skimming through other users 'dotfiles' or similarly named repos. It's a great way to find some small tips or shortcuts to make my day to day work more efficient, etc, without reading a long blog post about why some technique is the 'only sane, correct, right, proper way' and you'd have to be crazy to try anything else. I.e. great tips without the commentary.

The latest one I discovered is Slidedown: a simple tool to generate HTML slides (with syntax highlighting) from a markdown document.

Came in handy when I had to whip up a presentation for a recent meetup.

I'm a big fan of zen coding, it's a way of writing HTML and CSS by shortcuts and expanding them, cross-IDE: https://github.com/sergeche/zen-coding it has sped up my html writing considerably.

If you're using Django with a MySQL heavy application, consider django-stored-procedures (written by a colleague of mine) https://github.com/jeroeng/stored-procedures

Gordon; An open source Flash™ runtime written in pure JavaScript

EXAMPLE: http://paulirish.com/work/gordon/demos/ GIT: https://github.com/tobeytailor/gordon

qlmarkdown. It enables OS X to show (pretty/formatted) Markdown documents in Quick Look.

I also forked something called Add-Another for replicating parts of web forms (when you need to collect 0-n things from a user, e.g. emergency contacts, images, etc.)

AppSales-Mobile to an amazing and well supported find to track your iOS app sales. https://github.com/omz/AppSales-Mobile

On the Mac Brotherbard's Gitx fork is very cool. https://github.com/brotherbard/gitx

GitFlow - takes your Git skills to the next level. https://github.com/nvie/gitflow

Wukong, a Ruby-based Hadoop streaming framework. http://mrflip.github.com/wukong

What.js

https://github.com/entmike/What -- I've been playing with this, written by a buddy of mine. It's a Node.js-based application server, complete with an HttpServlet implementation.

I found isaac to be pretty cool. I had to fork it and add features because it's not maintained too well, but a neat idea.

https://github.com/ichverstehe/isaac

http://github.com/rohityadav/cmakeqt a qt-cmake example, with features like cross compiling, packaging, translations etc.

Vim-inspired file manager for the console

https://github.com/hut/ranger

Lots of goodies here: https://github.com/MarcWeber

self-plug: GithubFinder http://sr3d.github.com/GithubFinder

Much better code/repo browsing for Github repos, especially with the bookmarklet or the Userscript plugin.

libtelnet: A nifty little library that makes it easy to speak proper telnet.

https://github.com/elanthis/libtelnet