In Lithuania: Marijampole County

Marijampole county is made up of five district municipalities: Marijampole m., Sakiai m., Vilkaviskis m., Kazlu Ruda m., and Kalvarija m. If your ancestors hailed from this county, or if you just have questions about this area, discuss them in this forum.

Marijampole County page on Wikipedia.
Petras Kriaučiūnas Public Library of Marijampole website (in Lithuanian)

2001 Census
Mariampole County's rural settlements and their inhabitants (in Lithuanian).  This is a PDF file, and it's interesting, but you'll probably be using Google Translator a lot.

Churches and records:
At the end of WW II, the Cathedral in Vilkaviškis was destroyed. The Soviets razed it and the land became a public park. The same thing happened to the church at Pilviškiai. It appears that the Archives in Vilnius only have parish records for Vilkaviškis for the years 1795-1812. Those searching for more recent records from this area might want to try the Polish Archives in Suwalki.

Here is a list of parishes in the Diocese of Vilkaviškis.  The page is in Lithuanian.  http://vilkaviskis.lcn.lt/dekanatai/

Lithuanian Army Records for the Wars for Independence (1919-1920)
This site is in Lithuanian, but it is a valuable resource in finding
ancestors who stayed behind in Lithuania, or emigrated later.  It lists
volunteers for the Lithuanian Army during the Wars for Independence. 
The link below will take you to the names beginning with "A".   Just
click on the first letter of the surname for which are searching.

Fulfillment of military service earned the soldier free land.  The descriptions
generally give a certificate number, and where the land was granted. 
Birthdays and relative information may also be listed.

http://www.versme.lt/sav_a.htm