The main text for the ritual or sacrament of footwashing is found in John 13:14-15. Some Mennonites regularly practice footwashing, others do not. Many find it a meaningful way to recall the servant posture of Jesus and use this ritual as a way of aligning themselves with this attitude of Jesus. It is now done in the some modern Mennonite churches at the Maundy Service prior to Easter. The complete theological basis for the practice is found in the Confession of Faith in a Mennonite Perspective.
For a three and a half minute video exploring the practice of footwashing, including the scriptural background, order a DVD or download here.
For a longer explanation of footwashing see http://gameo.org/index.php?title=Feetwashing.