Gardening cheaply has me seeing my ziploc, tupperware, and gladware in a new light. Everything looks like it would make a great planter now. My current favorite is ziploc's larger containers, which we inherited with leftovers from thanksgiving last fall.
Preparing plasticware with a lid is the easiest since most nestles into the lid. Here's how I did it.
Step 1: Turn the plasticware over so the bottom is upright and use a sharp tool (an awl, scissors, a box cutter, a knife, whatever you have handy) to cut several holes or a few slits in the bottom. These holes will serve as drainage, so cut with care: too many/too large and the water will flood out, too few/too thin and the soil will become waterlogged.
The image on the right is an example of a slit and hole cut with the box cutters. You need only one of these in a small container (sandwich sized), and two or three in a larger one (think lasagna sized).
Step 2: flip the container over and set it on top of its lid. The bottom should nestle comfortably in the lid. Fill the container with soil.
Step 3: Secure the lid to the container. In this case, I've used duct tape. It's an inelegant solution, but since I'm only using to start the plants, it does not need to be attractive. Plus, it shows how universal duct tape is as a solution.
Step 4: Add soil and seeds to plant. Put your new pot in a sunny window and wait for the seeds to germinate.
I like the larger containers because they simulate the seed starters you can buy at the store and the garden environment.