There are a number of games which you can start up, play for a half-hour, and then set aside. Depending upon your mood, these can be some awesome games.
If you're playing World of Warcraft... you don't merely play for a bit, and go hang with the children. You're in. All in. You sit down at 8pm, and rip yourself away from the keyboard at 4am, hopefully with some nice treasure drops, and a level under your belt.
But what if you only have an hour? Or, heh, what if your attention span doesn't last that long? Here is where your pick-up games come in.
Lately, Mercenaries 2 has been my pickup game. Driving games also excel (heh) at being great pickup games. Shoot some bad guys, or run a race. Done, recorded. Go see your buddy for a nightcap whiskey.
I've been playing a lot of Sacred 2 over the past months. It does not require a "mindset" to get into, and it has a hojillion quests. It works very well for that casual-gaming / pickup atmosphere. On the other hand, my latest Dragon Age: Origins save game is right before some scary-ass demon bitch with a dozen conversation menus of choices about whether I decide to kill her or not. Oh, and if I fail to kill her? Then I gotta redo the menu sequence all over again. It is a "hump" that I am not looking forward to crossing over and over, so I haven't put that game back in the machine. Dragon Age is great, but it requires a mindset, and a time commitment.
Game style. What do you like? Do you have time? Can you find long stretches of game play? A game could be awesome, but it may require more head-time than you have available.
And with all that said, Mercenaries 2 is available, used, for less than US$20 at GameStop. Pick that up, and go blow up shit. It will take you 15 minutes to use a tank and reduce your enemies to rubble. And Merc2 is wicked about destruction. No holds barred. Adrenaline. ... Oh, then go kiss your wife goodnight. She doesn't need to know you're an avatar of wonton destruction. You can always start up Mario and whistle a tune, if you need cover...