Anyone who pushes their opinion of atheism on others. Just like any fundamentalist religion with the need to preach their belief systems to others in the hopes of converting others to their cause.
Bob preached his fundamentalist atheism when asked to bow his head in prayer at Thanksgiving dinner.
n. 1. Dogmatic subscription to atheism characterized by forcing atheistic beliefs on non-believers.
2. The height of religious irony
Richard Dawkins' fundamentalist atheism is hilarious since he's bashing fundamentalist theism.
While some atheists can certainly be annoying and intolerant as fuck, there is actually no such thing as "fundamentalist" atheism.
The definition of a fundamentalist is someone who won't be swayed by evidence - someone who relies on "faith". This is in contrast to the nonbeliever, who relies on hard evidence and reason.
And believe me, the second an intelligent nonbeliever finds a good enough reason to believe in any particular deity, he will do so immediately.
But what sort of evidence would convince a religious person that his faith is false?
Look up the definition of fundamentalism in dictionary, folks. "Angry" atheists are preaching for rationality and a respect for evidence, not faith.