Sven of midilooper has just released an impressive online MIDI Looper available for free. It's capable of looping sequences played from your qwerty keyboard or a MIDI input, using WebMIDI (Chrome), recording them on multiple tracks and sending them out to your MIDI devices or VSTs.
It's loaded with performance features for muting and soloing layers, as well as altering their respective velocities. You can map shortcuts to the keyboard to customise it's behavior - with functions to allow for recording on inputted notes, transposing octaves and sending program changes. Visually it's not quite Pro Tools or FL Studio but it's still early days.
Here are some of the specs:
It'll work with drums as well as synths, where both can sequenced in one place. Head over to midilooper.com to give it a go.