Clogged Drains: this won't work for serious clogs, but it's a low cost solution for a moderately clogged kitchen sink (it won't work with hair). First drop in a couple Alka-Seltzer tablets down the sink, and follow it up with a chaser of vinegar (1 cup). Allow for the chemical reaction to work its magic for about 2 minutes, then run the hot water at full force to help jettison out the debris. A bonus is your drain should smell better too.
Brighten up the Toilet: Alka-Seltzer works thanks to citric acid and baking soda, which is chemically released when mixed with water resulting in that characteristic effervescent fizzing action. Just a couple tablets dropped into a toilet will work on that porcelain in a similar fashion to denture cleaning tabs on false teeth. Just give them about 15 minutes to fully dissipate and work on those stains and scrub gently afterward for a whiter throne.
Clean Glass Oven Cookware: we learned this one in college while we lived in the dorms. A friend showed his cleaning technique after making lasagna for everyone. He dropped in a whole box of Alka Seltzer into the bakeware, fully filled to the brim with warm water. Soaked overnight, the stains were much easier to remove the next morning. Too bad we couldn't do the same thing to our whole dorm kitchen (yuck).
Clean the Microwave Dish: one of the nastiest, neglected areas we used to see in the office kitchen we used to work at was the inside of the microwave. Jonathan has listed how to clean the inside before, but we also recommend removing and cleaning the bottom plate that many rotational models use for even cooking. Submerge into the sink, with water just barely covering, then drop in 2-4 tablets and allow to soak for an hour. Scrubbing the stains will be 10X easier afterward.
Anybody else have some household uses for Alka-Seltzer you'd like to share?