I'm pretty familiar with the Game Engines out there for 2d games, for mobile. (Personally I love Corona for that).
But, for the Desktop PC side for 2d action games with multiplayer capability, I'm out of the loop on what indie devs are using these days. This is just prospective, kind of curious, not a pressing need but wanted to look into it and maybe start learning one in my off-cycles.
Unity3d (in 2d-mode) and Love2d look the best so far. Love2d looks like a much smaller learning curve, but it has 1/100th the user-base which is a big disadvantage. Anything else?