Update 2: I’ve posted an improved version of this answer to the Visualizing branch topology in Git question, since it’s far more appropriate there. That version includes lg3, which shows both the author and committer info, so you really should check it out. Leaving this answer for historical (& rep, I’ll admit) reasons, though I’m really tempted to just delete it.
My two cents: I have two aliases I normally throw in my ~/.gitconfig file:
[alias] lg1 = log --graph --abbrev-commit --decorate --format=format:'%C(bold blue)%h%C(reset) - %C(bold green)(%ar)%C(reset) %C(white)%s%C(reset) %C(dim white)- %an%C(reset)%C(bold yellow)%d%C(reset)' --all lg2 = log --graph --abbrev-commit --decorate --format=format:'%C(bold blue)%h%C(reset) - %C(bold cyan)%aD%C(reset) %C(bold green)(%ar)%C(reset)%C(bold yellow)%d%C(reset)%n'' %C(white)%s%C(reset) %C(dim white)- %an%C(reset)' --all lg = !"git lg1"
git lg/git lg1 looks like this: 
and git lg2 looks like this: 
(Note: There now exists much more applicable answers to this question, such as fracz’s, Jubobs’, or Harry Lee’s!)