AA: Harrisoburg Senators (currently 10-8 and 3rd in the standings)
Official Website (next game: April 29th vs. Akron Aeros)
Roster for AA: Harrisonburg.
Current Stats for AA Harrisonburg Senators.
Media Guide, containing 2004 stats and more.
None are on the 40 man roster.

A (Advanced): Potomac Nationals (7-11 and last in their division; last game hit 5 home-runs, won 9-0; April 28th's game is dollar night and it is currently 68 F in Woodbridge).
Official Website.
Roster.
Stats.
That E Chavez you might see on the stats, and who has played 16 games on this team hitting just .250 and with an on base percentage of only .297, is not Endy Chavez, but Ender Chavez.

A: Savannah Sand Gnats
Offical Website. (Their website actually has something of a player profile for the players, though not career stats; May 2 is Mullet Madness Monday ("Wear your best Mullet or wig"))
Roster.
Current Stats.

SS (short season?): Vermont Expos (season not yet started)
Official Website.

R: GCL Nationals (Season: not yet started)

Interesting examining the Washington Nationals' minors.