From GitHub blog:

GitHub Pages allow you to publish web content to a github.com subdomain named after your username. With Pages, publishing web content becomes as easy as pushing to your GitHub repository.

To create your own pages, follow these steps:

1. Create repository named you.github.com, where you is your username
2. Add some pages (index.html or index.md for example) to the repository and push
3. Wait a few minutes for your pages to be created for the first time
4. Visit you.github.com and see the simple yet amazing feature working

I’ve already created my own: ashchan.github.com

It’s time to learn the Jekyll site generator and play with it.

Update:

Dr Nic wrote up an awesome post: Migrating project websites to github pages with sake tasks, new websites with jekyll_generator

Update 2:

GitHub now offers Pages Generator: see here. Really cool! It took me 1 minutes to generate the project page for Gmail Notifr: http://ashchan.github.com/gmail-notifr/