I'm pretty sure UncleBob has played either Four Swords Adventures or Crystal Chronicles (or both) with five GameCubes, four with Game Boy Players, and five TVs. I love UncleBob, because he makes me feel like I've made sane decisions about how much stuff I've bought from Nintendo.
I've done a few fun things with my Game Boy Players.
Final Fantasy: Crystal Chronicles - Though I've never played 4-players, I have hooked it up (5 GCNs, 4 Game Boy Players, 5 TVs/Monitors).
Four Swords Adventures - I have had four players here - 5 GCNs, 4 GBPs and 5 TVs/Monitors.
Four Swords + - Same thing, just in the Tetra's Trackers mode.
Wind Waker - Second GCN/GBP as the Tingle Tuner.... for no reason except I can.

Pac-Man Vs. 2 GameCubes and 1 Game Boy Player. This one doesn't actually work as well - I always wanted to get one of those lazy-susans and sit the Pac-Man TV on it so it could spin around and face only one player.
Original GB Tetris - Did two fun things with this - Obviously, two GameCubes and two Game Boy Players. Nothing special. More fun was one GameCube, 1 Game Boy Player, 1 SNES and one Super Game Boy 2.

Four Swords - Only played 2-players more than once. Played 4 players one time.
Mario Kart: Super Circuit - Same as above.
Super Mario Advance - Mario Bros. Classic mode - same as above.
Animal Crossing: You know that cool mode where you can download the NES titles to your GBA? Works on the GBA player too, if you're playing on another system.
FaceBall 2000 - 16 Players. I've only actually maxed out at, I think 11 people. Really need to get 16 people together to actually play it. Obviously, I used other Game Boys in addition to the Game Boy Player.

F1-Race - one of the few 4-player games - copies of this are usually super-cheap, so I have four.

Here's the trick though - the regular four-player adapter won't work on the Game Boy Player. It has an original-style Game Boy ext. port (wide, similar to a USB plug) and there's no adapter that I've ever seen that lets you go from the wider-plug to the smaller plug (there is an adapter that goes the other way around). I had to have one of my spare 4-Player adapters (also found super cheap) modified with a smaller plug on the end.

Super Mario Bros. DX vs mode - fun, fun.

Generic WaveBird - so, on the NES Classics titles, you can "clone" the game to another Game Boy Advance unit when you link them together. For games like, say, Super Mario Bros., if you do this and play it in single-player mode, the second screen just shows what's on the first screen. So, take the GBA unit of your choice, plug a wireless adapter into it. Take you Game Boy Player, plug a second wireless adapter into it. Boot up Mario in either unit, link 'em, then sit back on your couch and play Mario on the big screen, using your GBA to control the action.

One of my favorite things about the Game Boy Player is that you only need the disc when you first boot the unit - after that, you never need it again. This has allowed me to pick up a few Game Boy Players without the disc - saving me some money to run these monster things.
