I've seen some books and articles have some really pretty looking graphs of Git branches and commits. How can I make high-quality printable images of Git history?

_{Update: I've posted an improved version of this answer to the Visualizing branch topology in Git question, since it's far more appropriate there. 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(auto)%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(auto)%d%C(reset)%n''          %C(white)%s%C(reset) %C(dim white)- %an%C(reset)'
lg = 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!)}