Animals I've seen not otherwise listed on this thread:
Mammals
Kangaroo Island short-beaked echidna
Stevenson's klipspringer
White-throated woodrat
Birds
Somali ostrich
Tongan megapode
Attwater's prairie chicken
Parakeet auklet
Madagascar buttonquail
Galápagos storm petrel
Galápagos shearwater
Galápagos petrel
Great frigatebird
Galápagos yellow-crowned night heron
Galápagos hawk
Cuvier's toucan
Galápagos mockingbird
San Cristóbal mockingbird
Small tree finch
Western large tree finch
Medium ground finch
Small ground finch
Large ground finch
Common cactus finch
Woodpecker finch
Vegetarian finch
Grey warbler finch
Green warbler finch
Reptiles
Upemba mud turtle
Kimberley River snake-necked turtle
Arakan forest turtle
Española Island tortoise
Galapagos leaf-toed gecko
Peters' leaf-toed gecko
Seychelles bronze gecko
Decary's leaf chameleon
Boehme's two horned chameleon
Marine iguana
Galapagos land iguana
Mophead iguana
Olmecan jumping viper
Mato Grosso lancehead
Angel de La Guarda Island rattlesnake
Usambara mountain viper
Sinai desert cobra
Obviously it's a rather odd assortment of odds and ends, and I know for a fact that other ZooChatters have seen some of these species. I have seen my fair share of species which were already posted (such as yapok, Sanford's lemur, and shield-nosed snake) so I figured I'd bring a new list up to the table for discussion.
Worst regret, miss or gut punch in the history of Zoochat!
Not quite

When I was about ten years old I visited Zoo Atlanta- at the time, exhibiting some of the world's last Rabbs' fringe-limbed tree frogs. However, I was relatively disinterested in herps at the time, and as such, I wouldn't have paid more attention to them than other frogs. I have no memory and no photographs of the frogs, so I will never know if I have seen this now-extinct species.