That's the GSG-5. It's the hottest little 22 on the market right now and it's harder than heck trying to get one (I've tried!). They go for under $500 unless someone is trying to price gouge you.
A lot of the airsoft and real MP5 items work on it too, like various buttstocks, foregrip units, scope mount platforms, etc.
Oops, got beat by 60 seconds!