gitg
Very intuitive branch model. You can select different branch, which will be displayed as the left-most line in the branch view panel. Cover the functions of giggle.
gitk
Come with git.
See all branches: gitk --all.
Not intuitive for branching information.
git gui
Install with aptitude install git-gui.
Use gitk's branch-browsing interface.
SmartGit
SmartGit is free for non-commercial use. You have to download the installer, extract it and run.
giggle
Install with aptitude install giggle.
You can browse the history of a file.
git cola
Install with aptitude install git-cola.
Like git gui, use gitk's branch-browsing interface, too.
Summary
gitg is the first choice. SmartGit is good except its license restriction.