Using your DEX mod for a trip attempt.

[Yucca]

Is there any way to use a DEX mod instead of a STR mod when making opposed checks to trip someone?

[OMG PONIES]

The Mighty Throw maneuver from Tome of Battle comes to mind first. Setting Sun discipline, initiator level 1. Also, the Trip Shot trick from the Master Thrower prestige class in Complete Warrior.

There are probably others, but that's all I've got right now.

[Eldariel]

In general, Tome of Battle - Setting Sun. The whole idea of the school is the whole "speed over power"-stuff with tons of Dexterity-based things (that could normally be Str-based). There's a large number of "throws" there that are mechanically treated as Str- or Dex-based Trip Attempts.