Sadly, most of them are not Last Man stories at all.
Main things to investigate:
There was a short-short in Omni some years back, and I can't remember the title. The last man on earth is a German boy whos aging has retarded. The last woman on earth is a Japanese girl with the same problem. They talk by radio, and the rest would be spoilers. Beautiful story. I think it was "Forever" by Damon Knight, but don't quote me.
Marooned in Real Time by Vernor Vinge has a last woman on Earth scenario. It's post-singularity rather than post-apocalypse.
Edit: Read 2008 May. Story doesn't fit into genre. She knows other people are alive, she just can't live long enough to see them again.
This is a running theme in all the incarnations of the Twilight Zone. In the second (1980s) series, the shattering "A Little Peace and Quiet."
Edit: Story is actually about a woman who stops time to prevent nuclear war, but then is trapped forever. If she unfreezes time, the missiles hanging over America will detonate. An altruistic re-tread of first Twilight Zone series episode A Kind Of Stopwatch.