Been playing lots of Global Agenda in the last week - very much worth a look. Sci fi 3rd person shooter with both PvE, and PvP. The games been around for a while so it's been well balanced, tweaked and populated now.
It's free with no level cap although you can pay a one off £14 for elite status which just means twice the XP/money and more loot at the end of missions (which I did after an evening of playing it).