This is the current build I have been going with for Trundle, this isn't a defensive build because I gather those items based on the other teams setup.

I love armor penetration, it seems to make the biggest difference. Health is always nice and by the time you leave the jungle the HP/lvl will be higher gain than the straight HP runes. I chose cooldown reduction because having all of Trundles abilities up is key.

There actually is a free point to be spent, Greed isn't required but it is a nice bonus. This setup allows for great jungling and ganking survivability. I do sometimes substitute the extra point on greed to pick up exhaust and remove the point from ghost into Good hands.

This is my normal progression, if heavy magic Force of Nature or Banshee's Veil are my favorites. Otherwise I stack Thornmail and Randium's Omen, or I mix and match. If a heavy AD team, substitute merc treads for ninja tabis. I gather defensive items before Bloodthirster usually.

YOu have to max bite whenever you can, it is your bread and butter and works great once you get Sheen for your Trinity Force. I tend to get 1 point in Pillar of Filth as that does the trick and then up Contamination whenever Bite or Agony aren't available.

The 3 I use the most are Ghost, Smite and Exhaust. If jungling(which is almost always) I just substitute exhaust and ghost depending on what I think will be needed with our teams setup, if we lack CC it's nice to have Pillar of Filth AND Exhaust.

Well this is my first build ever posted, and I know it's text heavy and not very pleasing to the eye, I will work on it more, but it is back to playing Trundle!