There are many transitional forms today. When we say transitional, I guess it means a spiecies wich gradualy changes it's way of life, usualy because of enviromental changes, predators, or something like that.
1) Polar bear http://en.wikipedia.org/wiki/Polar_bear Clearly a land quadripede, but hunts in water and spends quite a lot of time in water. Already developed some adaptations for cold water like greesy fur. This species might eventualy transform into:
2) Seal http://en.wikipedia.org/wiki/Sea_Leopard This is an example of mamal wich is gone half way trough transformation from a quadripedal carnivore to something like a fish-like animal. Another animal, but mammal, bird instead is going through similar phase:
3) Penguin http://en.wikipedia.org/wiki/Penguin Both penguin and seals are nice examples of "half-way" adaptation. Neither land or aquatic, they can both walk and swing, but aren't particulary good at either.
6) Walking fish http://en.wikipedia.org/wiki/Walking_fish Will it's descendants repeat again an epic jurney from water to land? If so, this fish is transitional form from a fish-like to land-crawling animal.
All above animals are nice examples because they're going through changes which already occured in past to other spieces, so we can clearly correlate and recognize various stages. But future will give us many more variations, wich we won't witness, and it's hard to immagine. It is safe to say that most of spiecies are transitional into something different. There are some examples, like sharks, who live in non-changing enviroment - open sea - who will probably look the same for a long time. But all others who live in always-changing enviroment will continue to change to best fit their habitat.
These are just examples from top of my head, if I think of something else, I'll post more.