I'm looking for a networking middleware that may be suitable for a medium-sized MMO.

I don't care in which language it's written, just that it's high-level, stable and has many of the features that can ease my development. I am making a 2D real-time action game.

Come to think of it... I'd be happy with "just" a high-level networking framework that has a few handy features to ease the development of a general networked game.