My dirty laundry
I'm pretty far from being the world's cheapest man (See my recent reality show: "Extreme couponing, it's not my bag, baby!") but I have recently started combining science, cooking, and cheapness into the great money-saving stew that is home-made laundry detergent.  As it turns out, this money-saving stew is also handy for getting beef stew stains out of your white shirts.  Stewpendous!

Caution, home-brew science ahead!

My recipe comes from The Simple Dollar.  That page has a whole bunch of commentary and pictures that you should check out, but in the interests of time here is the concise version:

5-gallon bucket with lid
1 cup washing soda (about $4 for a 55oz box - enough for at least 6 batches)
½ cup borax (about $5 for a 76oz box - enough for at least 18 batches)
1 bar soap (any kind - less than $1/each)

Boil about 4 cups of water on the stove.  I recommend dedicating a cheap pot and wooden spoon for this purpose so you don't have to scrub soap scum out of your favorite pots.  There's nothing worse than soapy pasta.

Completely grate the bar of soap using a box grater and then add a little of the shavings at a time to the boiling water.  Keep adding the soap and stirring until completely dissolved. Remove from heat.

Fill bucket with 3 gallons of warm tap water then add soap mixture to the bucket along with the washing soda & borax.  Stir until combined then cover and wait 24 hours.

That's it.  You should now have about 3 gallons of a sort of gelatinous goo.  If it looks & feels pretty gross but smells fresh and clean then you did it right.  Stir it up and use between ½ and 1 cup per load.  You may want to decant some to a smaller container for manageability (a clean gallon jug works well).

Each 3+ gallon batch should cost less than $2, exclusive of water costs (if you use Evian like I do then it might be a bit more) and should last you a good long while.  In my experience it works just as well as any store-bought detergent.

Now go forth and start your mad science of savings!