There were two, Colosseum and XD: Gale of Darkness (wow, what a stupid name), but they weren't that great.
Disregard the bold text. Colosseum and XD are objectively the best spin-off games of the franchise.
Mystery Dungeon begs to differ
As does Pokemon Conquest.
I have Colosseum, and find it extremely mediocre. The gameplay is just so painfully slow. If it wasn't Pokemon, I probably wouldn't bother. It is on the right track, though. --- I like Monster Rancher.