If you've ever considered trying City of Heroes, now is a great time. You can play to the level cap for free and team up with paid subscribers while doing so. Free players are locked out of some content, as is the case with MMOs on a hybrid model, but that content is available on the Paragon Market for reasonable prices. You have the freedom to pick and choose which content you want to pay to play.
What is most exciting for me, as a long time player (seven years and counting!), is that Paragon Studios has tackled the question of a sequel in a very elegant way. Without people really noticing, they built (and are still building) the sequel to City of Heroes right on top of the old game, and made a smooth and painless transition in the process without long time players having to start over. It's really quite wonderful.
The old story arcs have progressed, the new story arcs are moving the game forward, and the new tech of the game is familiar enough that returning players are going to feel like they're coming home.