There is a new Blockbuster Express rental code out for today only:
MONDAYDVD
This will work for one free rental per account.  You do need to return your movie tomorrow or you will have late fees.
These codes will only work on movies priced at $1.00- not the hot rentals priced at $2.99.
You can reserve a movie and enter the code to be sure it is working at Blockbuster Express.  I just tested it and it worked fine for me.