Just got Giana Sisters: Twisted Dreams free as part of the instant game collection on PS3's PSN+. I'd often heard about Giana sisters but never played it, most admit I quite enjoyed it, although the graphics were a little too good that I found it hard to marry up with being a retro game. After a while I just started enjoying the gameplay though... :)
Also, Stealth Inc is quite a fun 2D puzzle platformer...