Microsoft has recently slashed the prices of a number of their XBLA games as well as a few Games on Demand. A fairly comprehensive listis below:

Lode Runner – Was 1200 MSP, now 800. This goes along with the release of new DLC for the two year old title.
Penny Arcade Adventures: Episode 1 – Was 1200 MSP, now 800.

Penny Arcade Adventures: Episode 2 – Was 1200 MSP, now 800.
Grand Theft Auto IV: The Lost and Damned – Was 1600 MSP, now 800.
Games on Demand
Batman Arkham Asylum – Was $39.99, now $29.99
BioShock 2 – Was $39.99, now $19.99
Borderlands – Was $29.99, now $19.99

Dante's Inferno – Was $29.99, now $19.99
Super Street Fighter IV – Was $39.99, now $29.99
Paired up with the price drops that were announced back in December (Battlefield: Bad Company 2 and Burnout Paradise DLC) and the large title update for Hydrophobia that transformed it into "Hydrophobia Pure" (along with a price drop to 800 MSP, now is a great time to do some digital shopping.