If you encounter problems using GitHub on an Ubuntu system these hints may help you.
Typical symptoms include Eclipse failing with unknown exceptions and this sort of thing on the command line:
error: RPC failed; result=56, HTTP code = 200 fatal: The remote end hung up unexpectedly fatal: early EOF fatal: index-pack failed
GitHub oftens seems to use a version of git that is higher than the version distributed in the current LTS version of Ubuntu and this can cause problems if the git protocol has changed. At the time of writing the current Ubuntu LTS is 12.04 which ships with git version 1.7.8, but GitHub requires 1.7.10 minimum. (See  for the latest version requirements). To upgrade git you need to add git's package repository to the package manager then upgrade. This used to be easy until Unity came along, so now you have to open a terminal window and enter the following commands:-
sudo add-apt-repository ppa:git-core/ppa sudo apt-get update sudo apt-get upgrade
(you will be prompted for your password when you enter the first line).
When cloning a repository on GitHub you would normally copy the repository name given which has either an https: or an ssh: protocol specifier. For some reason these don't work and the solution is to manually change the protocol to git:. For example, the repository URI for the Joomla CMS should be https://github.com/joomla/joomla-cms.git, so try using git://github.com/joomla/joomla-cms.git instead.