Do you mean popular all over the country (like people travel there to race)? Or popular (sell out quickly) in the geographic region?

First case:

Kona Half (or whatever it's officially called)

Chicago Tri

Buffalo Springs

Pacific Crest 1/2

Phuket

NY Tri

really, any IM-qualifier

Second case for NorCal:

San Jose International

Uvas/South Bay Tri

Santa Cruz Sentinel

Pacific Grove

Big Kahuna (only in it's second year, but it got raves and sold out in the first year)

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clm
Nashville, TN
https://twitter.com/ironclm | http://ironclm.typepad.com

hmm..

Wildflower

Alcatrez (did any other non-IM get TV coverage last year)

Lifefitness (it'll be big this year, with live NBC coverage)

Chicago

That Kona half race thing

Tri-4-Fun Series or a Try-the-Tri, etc.

These are run by different groups all over the country, but the theme is the same - have fun. Great for first timers or a training race, or a friendly grudge match. A few thousand people do these races every summer just in Pleasanton, CA. They are a blast.