gitk
viewing all branches
gitk --all
viewing specific branches
Just list all the ref's you want to see. This is very helpful on large projects as opposed to showing all since so many commits and branches get out of hand if viewed together.
gitk origin/master origin/dev mybranch1 mybranch2
customizing appearance
At least on the Mac, the default look of running gitk is almost unusable. This is mostly to do with the fonts. You can edit the file at ~/.gitk to tweak these settings.
set mainfont {Arial 12}
set textfont {"Courier" 12}
set uifont {Arial 12 bold}