The Axlotl (sic) is probably the most well-known example. It's a member of the salamander's order, Urodela.
Although salamanders look rather like lizards, that's just due to convergence (see previous evolution post), and they're actually amphibians. Colin Tudge refers to that shape as the "archetypal tetrapod" - albeit one whose legs sprawl out the side, as in many reptiles, as opposed to being positioned beneath the body as with most mammals.
The axlotl is effectively a salamander that never made it to land. The adult form retains a number of the features of the salamander's early water-based existence, such as gills, while having grown the limbs that would be expected in an adult salamander.
Richard Dawkins (The Ancestor's Tale) details experiments that verified this neotenous nature - separately conducted by Vilem Laufberger and Julian Huxley (brother of Aldous). They each used hormone injections (thyroxine, the absence of which is said to have set off the neoteny), and induced the axlotl to lose its gills and become a fairly normal salamander.
Neoteny is actually the slowing down of other aspects of development relative to sexual maturity. This is subtly different from the acceleration of sexual maturity relative to the rest of the body, called progenesis.
Dawkins refers to the general case as heterochrony, which is the change of pace (a slowdown or a speeding up) of one developmental process relative to other developmental processes. He suggests that this must be behind a high proportion of evolutionary changes in anatomical shape.
Dawkins then mentions the newt, a type of salamander which does this twice, so to speak. It matures to a land-based adult form, but doesn't reproduce. It only becomes sexually mature when it returns to the water, and reclaims some (but not all) of its larval features. Strangely enough, one aspect that doesn't roll back is the gills. It's easy to postulate that the land-based form is not as redundant as it seems: it may have been quite useful at some point to ride out dry seasons - or, more plausible to me, it may have been evolutionarily advantageous to be able to travel a fair distance between ponds or other such water bodies. And it may have been easier to retain water-based birth practices (a somewhat harder call at why it switched the second time - it may have just reverted).
It is also speculated that humans underwent several differeny kinds of heterochrony - for example, the human brain keeps developing for several years after birth, which doesn't happen with chimpanzees. In fact, Wikipedia's Heterochrony entry claims (based on Penin, Berge, Baylac, 2002 and Mitteroecker, 2004) that humans demonstrate 30 different neotenies compared to chimps.
Dawkins, B (2004): An Ancestor's Tale. Phoenix, London.
Mitteroecker, P et al (2004): "Comparison of cranial ontogenetic trajectories among great apes and humans" in Human Evolution (2004) Volume 46, pages 679-697.
Penin, X, Berge, C and Baylac, M (2002): "Ontogenetic study of the skull in modern humans and the common chimpanzees: neotenic hypothesis reconsidered with a tridimensional Procrustes analysis" in American Journal of Physical Anthropology (2002) Volume 118, pages 50-62.
Tudge, C (2000): The Variety of Life. Oxford University Press, Oxford.