If you could bring back one Web site/service...
...that has ceased to exist, what would it be?
My vote goes to Grooveshark. It was a free music streaming service that shut down last year. I have yet to find a comparable alternative (and with the legal atmosphere being what it is these days, I don't expect one to show up)... even though some important names weren't available due to licensing restrictions, the rest of the selection was excellent, and diving through it made me find many, many great songs and artists.
For a runner-up, I thought of Megaupload or Rapidshare, except that if you think about it, on all occasions people bitched a bit over the lost files, uploaders switched to the second-best hosts, and that's it. Everyone moved on, and most people would be hard-pressed to name more than a few of all the services that have disappeared over the last five years.