Redbox and Blockbuster Express codes can be used one time per credit card number.
Free Redbox kiosk codes can be found here: http://www.insideredbox.com/redbox-codes/
REDBOX (new customers only)
Free Blockbuster Express dvd movie codes can be found here: http://insideblockbusterexpress.com/promo-codes/
Here are some of their codes from their list that are still valid.
GL9RA - valid until 3/31/10
GT21A - valid until 3/12/10
GOLIVE - valid until 3/31/10
G213A1 - valid on Wednesday, 3/10/10
GA13A2 - valid until 3/16/10
Free Blockbuster Express rent one, get one free rental codes:
GE12B - valid until 3/8/10 - expired
GN13B1 - valid until 3/8/10 - expired
POPCORN (do not know expiration)
Now go out and enjoy some free movies!
Update: teachFACSed on CheapStingyBargains has more deals here: http://www.cheapstingybargains.com/forums/freebies/35695-blockbuster-express-free-rental-codes-through-april-2010-a.html