Far Cry 2 and PlanetSide 2. No problem.
FC2 appears to be an open world Adventure-Shooter (I just made that up, but its based on ideas that were kicking around in my head ever since I tried to analyze Bioshock), and it is, for the most part. You get two discrete sandboxes to play in, and you can do whatever you want to, but mostly you'll kill a lot of nebulous 'bad guys' who politely re-spawn whenever you wander to the end of the block. This would be much more efficient if you had guns that didn't jam right away. Better quality weapons can be purchased in a limited in-game economy. You can earn currency through doing missions as a mercenary - and killing more nebulous 'bad guys'. Also there is a small amount of currency in hundreds of caches in the sandboxes, but scraping up gun funds that way takes much longer. You'll probably end up using about 6-8 different weapons, and its possible to spend all the money you need to spend before you leave the first sandbox - meaning you end up with a lot of excess currency. The number of missions you can perform are limited, and whatever story there is gets pushed slightly forward when you've done a certain number. Also you get buddies who can rescue you when you fall in battle, and also grant you perks if you do missions for them (also limited). The story itself is interesting, but not very engaging; the main supporting character talks too fast, and is a self-righteous blowhard. Interestingly, the game itself can raise player awareness of the horror of tropical diseases and poverty in Africa - but not to the point where they will volunteer for the Peace Corps. I liked FC2 almost immediately.
PlanetSide 2 is a "Free to Play" team-oriented, class-based, Massively Multi-player Online Murder Activity Playset for Sado-Masochists. That is to say that it has a lot in common with the multi-player versions of most of the SpunkGargleWeeWee games. You join one of three factions and have to go kill the other two factions because they are different colors than you. Friendly Fire is always on, and it can be difficult to identify the OpFor quickly - especially if you have toons with multiple factions. The world/island/thing that you are on is plenty big, and there are two more of them if you get bored with the one that you are on. There are some F2P shooters where your sidearm matters a *LOT*, but surprisingly, this isn't one of them. Your standard issue weapon is fairly good compared to others that you can unlock. You unlock weapons, attachments, upgrades and perks with certification points; which you earn by killing, healing, repairing, resupplying or hacking things. You gain cert points fairly quickly (teamwork really, really helps!) but those things that you need cert points for, take a LOT of darn points. AND you have to certify or unlock items SEPARATELY FOR EACH CLASS! If you want your tank to carry more ammo, you have to unlock that separately with each class that you intend to drive the tank with. So there's no such thing as spare currency in this game. There will always be something that you want to spend it on. If you can't bear to wait, you can spend money to buy Station Points, which can be used to unlock weapons, but interestingly, they are not entirely fungible with cert points. Approximately once a month(?) there are double- or triple- Station Point sales, where the same dollar amount gets you double the number of SP. (This may not be relevant, but SP seems to cost significantly more in the EU than in America. It's usually the other way around.)
PS2 has decent platoon & squad-based communication, and players can join 'Outfits' if they so choose, so the organizations are pretty SpunkGargleWeeWee standard. PS2 has what seems to be a large number of servers, and even the servers that are crammed full seem to run fine for me - on my 4 year old E8400 Wolfdale rig. After my first session of playing PS2, I was aggravated, and just barely resisted a strong impulse to uninstall it. Now I'm kinda hooked, so I should probably uninstall it for my own protection.
To summarize, lets consider it a checklist.
> Both open world.
> Both have neat fast-travel systems, both of which suit their setting quite well
> Both have ground vehicles that steer fairly well - much better than the cars in Saints Row 2
> Both require killing tons of dudes
> Both have formidable opponents, as FC2 has good AI, while the players in PS2 may be more clever than you (Tip: remember that PS2 has jetpacks!)
> Both will have the player purchasing upgrades
> Both have unique currency that can be acquired at reasonable speed
> Both can be addicting following successful forays, causing you to spend more time than intended
> Both end badly. FC2 has a stupid ending. PS2 ends when you uninstall it to get your life back.
However
> FC2 has invisible walls that you won't discover, unless you try climbing or wandering out in the desert. While in PS2 there are none, all the objectives are within your map, and no real reason to wander off.
> In FC2, you get to shoot absolutely everything that moves. The game does not allow you to hold weapons when it does not want you to shoot friendlies. In PS2, Friendly Fire is always on, and you will kill teammates. Just accept it.
> You pay for FC2 once, unless you get sucked into buying DLC. You will be strongly tempted to spend money on PS2, but you really can manage without. Maybe. The certification system is a little overbearing, but it doesn't constitute "pay to win"
> FC2 has no aircraft, while in PS2 you cannot run over wild animals. Players, yes.
Note: By Adventure-Shooter, I don't mean its related to point-and-click "adventure games". Rather, I mean a game where you kill things in an environment that is large, alien, or otherwise fascinating to explore. Bioshock would have grown wings and flown away if it was set in a larger explorable world.
Also: For Twing-Twang!