Well, it was the actual name of the novel.
"I am Legend" by Richard Matheson (1954), novel. This is a landmark novel in terms of Vampire Literature because it was one of the first to try and scientifically explain vampirism. It inspired George Romero's "Night of the Living Dead."
"The Last Man on Earth" starring Vincent Price (1964), b/w movie (script by Richard Matheson). This is a fantastic movie. It really haunted me when I saw it as a child.
"The Omega Man" starring Charlton Heston (1971), movie (script by John William and Joyce Corrington). I haven't seen this version; it deviates substantially from the source material.
Now, "I am Legend", with Will Smith (2007). I am looking forward to seeing this one.