I love sandbox games. The ability to do whatever I want makes the game highly attractive and replayable. My favorites of those genres are Minecraft, the Assassin's Creed series, and the Infamous series.
Minecraft is one of my favorite PC games. Like you said, sandbox games just never get old, but I prefer playing with people, not alone. Having to collaborate with someone to get the work done in Minecraft makes it more enjoyable for me.
I was actually looking for some buddies to keep me interested in Minecraft, everyone I know has gone into a more or less hiatus. I'll check out the servers :3
nerd.nu is a group of 3 reddit owned servers. They have a PvP, a PvE, and a creative server. I personally use the PvP server mostly because I like surviving without fighting other players. I go on the creative one once in a while to see some of the cool stuff they build.