Egg cartons are great for shallow rooted seeds that can go straight into the garden. They'll disentigrate after you plant them.
Salad bar boxes and other plastic containers with clear tops. You can start lots of seeds in these and then pick them out to put into individual growing cells. They top will hold in the humidity just like a greenhouse.
Newspaper You can roll a strip of newspaper around a form, such as a round spice container (small) or a soup can. Fold in the bottom and secure it with a strip of masking tape, and the container can plant right into the garden and disintegrate. The great thing is you can customize both the height and the depth of the container by using different size newspaper strips and different size molds.
Plastic Margarine Tubs with holes poked in the bottom for drainage. Use them for seeds and cuttings.
Cups Use paper or styrofoam cups. Just poke a hole in the bottom for drainage. Also old pie tins, or foil pans are good to use.
For a budget heat mat, you can buy a plumbing heat tape, a mortar tub, and a couple of bags of sand at the local hardware store. Put the heat tape in the bottom of the tub, cover it with sand, place your containers on the sand, and your seeds will have a toasty warm environment in which to germinate. Those giant sweater boxes with the clear tops that you can find at some discount stores make great containers for the sand, too, and they have a clear top to hold in the moisture and let in the light. Even though a plumbing heat tape is designed to be used in a moist environment, make sure that the outlet you plug this into is GFCI protected. The NEC states that any electrical outlet in a moist location should be GFCI wired, and a GFCI outlet is all of about $12 at your local hardware store. If you have lots of your outlets on one circuit, a single GFCI outlet on the first outlet inline will protect the whole string. Cheap insurance that may save you.
Take a 2 liter soda bottle and cut it almost in two about 5" up from the bottom. Leave about 2"-" uncut for a "hinge". Fill bottom 3/4 full with soil mix, and your favorite seeds or cuttings and close the bottle back up on the "hinge". Secure with a bit of packing tape. You can help control the humidity inside the greenhouse by screwing on the top or leaving it off. Do be sure to rinse the bottles good before trying this!