I'm looking for fun multiplayer games of any kind to play.

Well, there's the obvious choice in Smash Bros. Melee.

There's also Pikmin 2 which has a pretty unique multiplayer mode.

I like Custom Robo, though it's pretty weird and the multiplayer isn't really the draw. I enjoy it, though.

If you can get the batshit crazy 4 GBA connector cable setup going, Zelda: Four Swords is a good one.

On that note, so is FF: Crystal Chronicles.

The Gamecube has my favorite version of Soul Calibur II.

All of the Mario Sports games are super fun to play.

Super Smash Bros Melee

Mario Kart Double Dash

Wario Ware Inc: Mega Micro Game$

Mario Power Tennis

Super Monkey Ball/2

F-Zero GX

Zelda: Four Swords Adventures (if you have the required GBAs, link cables, gamepacks)

The Mario Parties (maybe)

One Piece Grand Battle/Adventure (basically Powerstone)

Timesplitters/2

Custom Robo

Kirby Air Ride is a fun time. There are still not that many games like it.

This, so much!!! The GBA connection setup is incredibly inconvenient, but I had a really good time with Four Swords and Crystal Chronicles. Also, if you somehow have more TV's and GameCubes with Game Boy players than GBA's, you can set up something like a crazy LAN party with those games.

What the fuck?

Phantasy Star Online.

Here's one I feel like only I played. It was a small game that has kinda been long since forgotten. I doubt anyone else is gonna put it on their list:

Super Smash Bros. Melee

Lots of good suggestions here, I would add NHL Hitz 2003. Like Blitz, but with ice.

Another vote for F-Zero GX. It's never too late to send Nintendo the message

Mario Kart, one of the most well regarded ones even.

Pikmin 2 has one of the coolest multiplayer modes I've ever played. It's quite unique and a ton of fun.

I'm probably in the minority on this, but the Clone Wars video game for the Gamecube had some really fun multiplayer modes. There was a really neat vehicle-based mode called Conquest that involved capturing control points. However, you could go beyond simply capturing the point and stay there longer to have it build turrets up around it. The game also had a Horde mode 4 years before Gears of War came out. My friends and I played the hell out of that game when we were 12-13.

Crush Hour

We always found sega "Beach Spikers" to be a really fun and underrated multiplayer game.

@getz:

I agree GB let me down taking so long for no one to mention the best multiplayer game on the cube. If you can find it try and get pso episode I & II plus but it's still super pricey last I checked.

I'm not sure more fun can be had on the cube than playing pso with three friends as your stupid HUnewearl buddy keeps getting one shotted by olga flow while my FOmar with his super easy to obtain Skyly god hps is doing just dandy! :p (also maxed out his hp mats as well)

Super Smash Bros. Melee (best game in the series yo)

Mario Kart Double Dash (actually felt more unique than the other games in the series with 2 people to every kart)

Final Fantasy: Crystal Chronicles (assuming you can find the crazy GBA and cables setup)

Soul Calibur II (man my friends and I had fun with this back in the day, and somehow the GCN's controls work. Also Link's in it!)

Mario Party (it's ok I guess)

Skyly always got the really good drops.

If you can find the Mary Kate & Ashley game, Sweet 16, then that. If not, I guess Mario Party is a budget alternative that you might be able to enjoy.

Super Monkey Ball tho. Monkey Target all day and all night son

Timesplitters 2. Flame tag!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

TimeSplitters 2, and while I didn't play three on Game Cube, I'd still say three. There are others, but TimeSplitters is the one that comes to mind most.