Trip for free feat.

[STR Ranger]

Just browsing the feats section of the SRD.

Breakguard- lets you get your attack back when you successfully disarm (need to be dual wielding)

Shield slam- Grants a free bullrush when you shield slam.

Not seeing an equivalent feat for free trips.
Felling smash is sorta cool, but doesn't work with a full attack.

Anyone know of one?

[chaoseffect]

Vicious Stomp is the closest thing I know of to what you're asking, as it lets you make an AoO against a tripped enemy, but it can only be an unarmed strike.

[Kazaan]

Well, any ability that grants a melee attack can be turned into a Trip, Disarm, or (according to a recent FAQ) a Sunder. That's probably why they didn't feel the need to add in additional ways to proc trips specifically. Understandable for Bullrush, Grapple, etc. but Trip functions a little different.

[RumpinRufus]

You can take a one-level dip into Maneuver Master Monk and get Flurry of Maneuvers. You can use it with any weapon (not just a monk weapon,) and use it even while wearing heavy armor.

[STR Ranger]

Just looking at addding archetypes to my TWF

Fighter guide.

Thinking of adding Lore Warden. Gets huge bonuses to CM's and Combat Exp for free.

My TWW advice advocate either a Breakguard build or Eldritch Heritage.

Felling Smash won't work because it takes away your full attack and won't work with Doublestrike.

Trip is an awesome move. Maybe not worth giving up a full attack for.

[STR Ranger]

Probably gonna add DUELIST VAMBRACES to the guide.

[STR Ranger]

Also can't find the Duelist magic weapon property from the PSFG

[chaoseffect]

You can make a trip attempt in place of a melee attack, so you can trip as part of your full attack (I think).