We've been playing some co-op games on the Wii. We still don't seem to have found anything great, but some of it has been okay:
Secret of Mana
This is not as great as I remembered it. The combat system is very frustrating. You walk around and attack with your weapon, but when you hit something there's some hidden to-hit roll going on, so against tough enemies and bosses you miss most of the time. But there's not even any feedback to let you know you were aiming in the right place. Then there's magic, which never misses and does huge amounts of damage, but you don't have many magic points to spend. Enemies get magic too, and there's not much you can do other than grin and bear it and heal yourself afterwards. Most of the bosses are just a question of whether you will run out of magic points before they die. Also, using magic and items pauses the game, which isn't a lot of fun in multiplayer. Oh, and the plot is confusing and nobody ever gives you reminders if you forget what you were supposed to be doing.
Final Fantasy Crystal Chronicles: Echoes of Time
Has some promise, but is hard to get into. For a start, you need a copy of the game for every player. We're playing with Keirstan on the Wii and me on the DS. One player (whoever is on the Wii) is the leader and gets all the plot and conversations. The other player just gets a lot of disdain from the NPCs who will only have a proper conversation with the leader. The game is very complex and has no tutorial. It's not a pretty game. I'd say that while Secret of Mana may be much lower resolution it's much clearer and prettier than this.
House of the Dead Overkill
This is hilarious. I really like the whole awful horror movie theme, with the cheesy voiceovers, sloppy cutting, grainy quality and ridiculous dialog. It's probably been my favourite of the co-op games so far, which is a bit of a shame, because I really wanted an RPG that we could enjoy together. We didn't get the Wii zapper despite the guy at Game insisting that the game would be "much harder" without it. I still have no idea how it is supposed to help.