Well Timesplitters 3 had a fairly long single player campaign, took even longer if attempting it on hard -_-. But yeah that and all of the challenges and such, this makes TS3 a fairly long game singleplayer wise.
Also what about GTA San Andreas, as most GTA's, allows you to keep unlocking more items once you've completed the main story line, although you'd hardly use them unless you wanted to go on a rampage (to the extreme)
Also DOW2, before I am cut down by the merciless people of the forum hear me out, once you unlock the deployment for the very last mission the game there are many other missions that you can choose to do before completing the campaign. Anyway the game will just keep recycling missions on this final chapter to allow you to unlock most of the wargear. This can take forever usually (imagine it on primarch difficulty,HELL)