Everyone will have different players in mind, but that's what's fun about the underrated.  It's also a great way to get introduced to more B-ball history.

My all underrated team:

C Artis Gilmore (Good article here)

PF Maurice Lucas (original enforcer, career double-double guy, all-defense selection or two, smart player)

SF Bernard King (proved himself a franchise player with New York.  Injuries cut his career short, though he made a comeback for a couple years with the Bullets)

SG Mitch Richmond  (awesome shooter, scorer.  Building block, but played with Sacramento for most of his career.  20+ ppg career average.  Kevin Martin is a sort of second coming of Mitch  
is one way to describe Mitch Richmond.  At least as far as I can tell; pookeyguru could probably give more insight to my claim if he is still around)

PG Jerry Sloan.  Yes, THAT Jerry Sloan (six-time all-defense team; the original Bull.  Led Chicago to the playoffs during their early years, including a game seven conference final against Golden State, which they lost by only 4? points.  Done in by injuries and gone from Chicago before Artis Gilmore showed up.  
Plays like he coaches.  Molded Stockton into a guard not unlike himself; don't let the disparity of assists in the stat sheet fool you)

Next question: possible Hall of Famers.  I don't mean shoo-ins, either.  Guys on the bubble, or possibly getting in through the veterans comittee (more discussion and interesting names with the scope of this question).  

I think Dikembe Mutombo could make the hall on a later/veterans ballot.  The DPOY awards, double-doubles through most of his career, plus humanitarian work and indelible image of him holding the ball in '94 after beating Seattle (the kind of stuff that seems purely rhetoric, but can speak to voters) just speak to me as slipping in later, maybe waaay later.

I'm not sold on Vlade Divac, but his historical value (pioneer in the passing, european big-man) may get him a better shot than I believe he has.  Probably won't see the hall, though.

An interesting one to watch will be Alonzo Mourning.  His career looked to be hall bound (though not necessarily first ballot) until the kidney problems.  Though I wouldn't consider him near the kind of talent Bill Walton was, he does have sort of the same angle (dominant, cut down by recurring problems, key bench player on a title team), so he might make it somewhere down the line.  

Any other players you can think of?