OK so I am a few days late announcing this, but I forgot, sorry :P
I would say that of all the free to play games out there these days, it's Star Trek Online that gives you the most content for free. Seriously folks this game pretty much throws everything at you for free and is very reasonable in what it expects people to pay for. It's also, in my mind anyway, the most improved since launch MMORPG probably ever.
As a free download this is one of the easiest recommendations I've been able to make.
Official site: http://www.startrekonline.com/