How can I view a git log of just one user's commits?

This works for both git log and gitk - the 2 most common ways of viewing history.
You don't need to use the whole name:

git log --author="Jon"

will match a commit made by "Jonathan Smith"

git log --author=Jon

and

git log --author=Smith

would also work. The quotes are optional if you don't need any spaces.

Add --all if you intend to search all branches and not just the current commit's ancestors in your repo.

You can also easily match on multiple authors as regex is the underlying mechanism for this filter. So to list commits by Jonathan or Adam, you can do this:

git log --author="\(Adam\)\|\(Jon\)"

In order to exclude commits by a particular author or set of authors using regular expressions as noted in this question, you can use a negative lookahead in combination with the --perl-regexp switch:

git log --author='^(?!Adam|Jon).*$' --perl-regexp

Alternatively, you can exclude commits authored by Adam by using bash and piping:

git log --format='%H %an' | 
  grep -v Adam | 
  cut -d ' ' -f1 | 
  xargs -n1 git log -1

If you want to exclude commits commited (but not necessarily authored) by Adam, replace %an with %cn. More details about this are in my blog post here: http://dymitruk.com/blog/2012/07/18/filtering-by-author-name/

git log --author="that user"

On github there is also a secret way...

You can filter commits by author in the commit view by appending param ?author=github_handle. For example, the link https://github.com/dynjs/dynjs/commits/master?author=jingweno shows a list of commits to the Dynjs project