For my money, the most underrated goalie in the NHL is Kari Lehtonen. He is yet another example of a very good goalie getting overlooked because of the poor team around him.
Lehtonen has gone 75-61-14 on Atlanta over the past 4 seasons, with an impressive .913 save percentage and a 2.82 GAA. That is much better than his backups have done (.890, 3.31). His save percentage is trending upwards - .906 in 2005-06, .912 in 2006-07, and .916 in 2007-08 even though the team took a substantial step backwards (although their defensive play probably improved). Hockey Numbers had him a solid 9th in the league this past season shot-quality neutral save percentage, and in 2006-07 Alan Ryder ranked Lehtonen as the 3rd most valuable goalie in the league behind only Luongo and Brodeur.
Lehtonen has posted very consistent even-strength save percentages - .930, .926 and .927 over the past three seasons, according to NHL.com. That level of even-strength performance puts him in a class with Kiprusoff, Giguere, Thomas, Luongo, Lundqvist, Vokoun and Brodeur, i.e. the league's elite goalies.
Given those impressive numbers, it is curious how poorly he is rated around the league. The Hockey News did a ranking of the top 30 goalies last spring, and gave Lehtonen the final spot in their ranking, #30, behind every other starting goalie in the league. There was also a recent series of polls on HFBoards asking fans to rank the starting goalies, and Lehtonen came out in just 20th place. I think Lehtonen is probably a top-10 goalie in the league today.
Atlanta will probably be a weak team again this season, but given his recent history and young age (24), expect Lehtonen to continue his strong play and gradually gain more recognition as one of the top young goalies in the league.