How can I add thermal mass to my house?
I have about 50 55-gallon drums stacked in my basement, all filled with water.  They are stacked one on top of the other (bottom row/top row) along our north basement wall.  I have two in floor-mounted fans, one pushing, one pulling, mounted in the ground-floor joist cavity (from above they just look like floor mounted cold air returns).  During the day, the fans circulate the air around the house.  Our furnace has a bypass door on the cold-air return that opens a door into the basement cold-air return.  This way, during the summer the furnace fan (on 365 days a year) pulls all the intake air from the basement.  With so much air coming from the basement, our 2 to 3 ton AC units run maybe 6 days in the entire summer.

The system is cheap to run (about 1/4 of our cooling costs with just the AC units).  I got all the barrels for about $200, and I have emergency water for years if i need it.  Plus, my house won't blow away in a gale.