Swashbuckler, Healer, Marshal, Warlock.
I strongly believe the Swashbuckler will be a core class in 4E. The mantra was always that Fighters could model any sort of fighting man, from archers to swashbucklers, but the truth is that, at the end of the day, it's best at emulating a big guy covered in armor, which is incompatible with a number of concepts.
Healers are a great, focused class, especially as Clerics have continued to expand and branch out. Clerics have become more interesting over time, but sometimes, damn it, you just need a medic. (And before anyone says that "no one likes to just be a medic," I can't get my wife, who's not exactly the Queen of Nurturing, to NOT play a medic in MMORPGs. You never can tell.)
Marshals do something no other class (or even prestige class) models well, and is a great addition to D&D and parties in general.
And I find Warlocks to be very satisfying flavorwise as well as being more interesting mechanically than the Sorcerer, which I find to be deeply boring as a class.