I can't say I know of any free anime conventions in the Maryland area. Are you looking for a big event or just a place to meet other anime fans and watch some new things?
On April 1st, the Freer and Sackler Gallery of Art, part of the Smithsonian in DC, will have an anime showing of 35mm films throughout the day. Admission is free though you have to get there ahead of time to pick up a ticket.
http://www.asia.si.edu/events/index.asp ... &year=2006 This event is being co-sponsored by Otakorp, Inc., (the non-profit company that runs Otakon as well) as part of the National Cherry Blossom Festival.