1. Firstly, it is dex damage, and you could do just as well with maximize spell as with twin, oh well. Divine metamagic is used to effectively add the metamagic feat for free, all in all not as useful as doing DMM Persistent Spell or DMM (anything) on an already long-lasting buff. The spell itself is regularly 3d6 dex damage with a touch, as I call it, "the dragon slayer." Really, the spell itself is the broken part of this "combo."
2. The power is used to turn into a Hydra and the feat lets you use its Su ability. You need the 12 headed Hydra for that much damage, though, and arguably each set of breath attacks applies resistances to the energy damage separately, so it's of questionable use.
3. Not sure if this actually works, but if it does... you're using a level 4 power and an easy to get feat to become a pit fiend. Not much more needs to be said.
4. Celerity lets you cast another spell as an immediate action. I think you lose your next standard, though, so whatever you're doing it'd better finish the job...
5. I don't know.
6. Iron Heart Surge is not broken, nor a combo. It does a LOT potentially, but mostly it just gives non-casters a prayer of surviving a fight with a caster. It's a level 3 ToB maneuver that lets you end one non instant effect or condition afflicting you. You could stop poison from doing secondary damage, end a fear affect, yell at an evard's tentacle field and make it go away... even punch through a force cage or more amazingly, an antimagic field (!!!). The "broken" part is usually because it doesn't just end the effect for you it out-right ends the entire effect. Like a dispel magic that always wins. Still, it costs a standard action and requires two feats, one feat + 3000 gp item, or Warblade levels in order to use (and in all but the last case, only once/battle) and mostly just screws over the more powerful classes anyway, so I don't mind it. And it on its own is not a combo.
7. Pretty simple. Craven (Champions of Ruin) adds +character level to SA damage. Staggering Strike (CAdv) causes every SA that deals damage to force a fort save vs. damage dealt or be staggered for one round. The synergy is blatant. And each feat on its own is extremely powerful, arguably broken.
Amendment taken on the Dex damage for Shivering Touch. I'm a dope.
Synchronicity is the psionic equivalent of Celerity.
And, there is a feat that lets you ignore the daze effect of Celerity.
Finally, remember that Celerity is an immediate action. Do it first thing in combat, and never lose initiative again. And if you can get it to be recursive, you can end every fight as soon as your DM says "roll initiative."