I vote for Carpool in north Arlington. http://www.gocarpool.com/
It's also real close to the Orange line if anyoine wants to Metro in from an outer station.
Directions
By Metro: Take the Orange Line to Ballston. When leaving Ballston station, turn left at the top of the escalator until you see the International House of Pancakes and walk in that direction. CarPool is one block past the IHOP on the same side of the street.
Driving from DC, you have two options:
1) Take I-66 West to Exit 71 (Glebe Road). At the top of the exit ramp turn left onto Glebe Road. At the third stoplight turn left — this is Fairfax Drive. CarPool is on the right just past the fourth stoplight.
2) Go over Key Bridge into Rosslyn. Make a right onto Wilson Boulevard and stay on Wilson through Courthouse and Clarendon until you reach Ballston. When you come to the JC Penney store in Ballston Common mall, turn right on North Randolph Street and then right again at the second intersection, which is Fairfax Drive. CarPool is on the right just around the corner.
From Alexandria and the I-395 corridor:
Take I-395 to Glebe Road North (Route 120). Then take Glebe North for about 3 miles until you come to Ballston. In Ballston turn right on Randolph Street, which is immediately before the Ballston Common Shopping Mall. Go through two stoplights on Randolph and turn right at the third stoplight. CarPool is on the right just around the corner.
From Maryland, Fairfax, and the Beltway:
Take I-495 to I-66 East (heading towards DC). Follow I-66 East to Exit 71 (Glebe Road/Fairfax Drive). The exit ramp runs straight into Fairfax Drive; there is no turn. CarPool is on the right just past the sixth stoplight.