I stumbled onto their website about 6 months ago, while doing a Google search on an ancestor, and now I find myself strolling cemeteries on my lunch hour, photographing tombstones to contribute back to the genealogical community.
You can create a memorial even if you do not know in which cemetery your relatives are buried. You can also leave 'virtual flowers' and upload photographs and biographical information.
Originally designed to be a site where you could locate graves of the rich and famous, many researchers also use FindAGrave to enter internments from smaller, family cemeteries. I've had some success in locating photographs of tombstones in distant cemeteries. When the opportunity presents itself, I reciprocate in kind.
Check out the listings of the virtual cemeteries I've created for my branches of the family tree.