Species:

Trachemys scripta

Pond Slider

Some other names for this species:

Jicotea

Subspecies I've seen:
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Red-eared Slider
T. s. elegans 		thumbnail
Mesoamerican Slider
T. s. ornata 		thumbnail
Yellow-bellied Slider
T. s. scripta
Subspecies:

Trachemys scripta elegans

Red-eared Slider

Sighting:
January 26, 2004
Sabine National Wildlife Refuge, Cameron County, Louisiana
Red-eared Slider (Trachemys scripta elegans)
My wife Monica spotted this large basking turtle twenty feet or so off the trail. I would have tried to get closer for a decent picture, but (A) I would have had to go hip-deep in swamp water; (B) the turtle would have slipped away when it heard me splashing; and (C) the several large alligators in the area might have come by to see what all the commotion was about.
Sighting:
March 31, 2004
Red Road Linear Park, Miami-Dade County, Florida
Red-eared Slider (Trachemys scripta elegans)
Young red-eared sliders like this one are very popular in the pet trade. Unfortunately, large adult red-eared sliders are not so popular, and they commonly get dumped into ponds, lakes, and canals around the country and indeed the world. They aren't native to Florida, but there are plenty of them living there now.
Subspecies:

Trachemys scripta ornata

Mesoamerican Slider

Sighting:
April 18, 2001
Gamboa Rainforest Resort, Panama province, Panama
Mesoamerican Slider (Trachemys scripta ornata)
The outdoor dining patio of the beautiful Gamboa Rainforest Resort overlooks a calm section of the Chagres river, packed with these turtles and a few caimans. The hotel staff tossed an occasional hunk of bread into the water, provoking a turtle feeding frenzy (as with sharks, only less frightening).
Subspecies:

Trachemys scripta scripta

Yellow-bellied Slider

Sighting:
February 4, 2004
Fairchild Tropical Garden, Coral Gables, Miami-Dade County, Florida
Yellow-bellied Slider (Trachemys scripta scripta)
This youngish turtle was relatively brave, allowing me to creep to within a few feet for this closeup.

I had originally identified this as a Florida Red-bellied Cooter. Mark Johnson wrote to point out that the large yellow ear patch identified this young fellow as T. s. scripta instead. These turtles aren't native to South Florida, but neither are most of the other herps you run across there.