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Some folks say Planetside 2's graphics engine (ForgeLight) is pretty intense. I haven't had a "weak" system to compare it against, but it'll run my FPS pretty low in large-scale fights. Pretty easy to find a larger fight, too--just hit "Instant Action" once you log on.
War Thunder is pretty nice looking at times if you want variety in results.
Path of Exile will bring your system to its knees with particle effects (but you don't really start seeing them spammed enough until high levels).
Hawken / Marvel Heroes will show you how online games + asset streaming / on demand loading will kill your system regardless of graphics card
There are several "traditional" mmo's out there that are absolutely beautiful and a great benchmark, dozens if you count korean mmo's, all F2P.
Rift on High settings really easy to check FPS just mouse over the PC icon on the corner but Storm Legion Zones is where it takes a dip.
or
Firefall on high settings
Trine 2 has a free demo and it's been known to burn up GPUs.