Heat tape (most commonly referred to by it's name brand "Flexwatt") is another common product used with reptile enclosures. This functions much in the same way as heat mats do since it provides heat from the bottom up, but it can provide you with a lot more versatility.
Heat tape comes in different lengths and widths, so it makes it easy to heat large areas or multiple enclosures simultaneously. Simply choose the length and width you need.A word of caution:
Flexwatt heat tape was not made with the intention of using it for reptiles. Flexwatt heat tape is for heating pipes and drums. Reptile owners have adapted it for their setups with success, but as with all heating devices, you must proceed with caution and safety.
Why safety? Because you will have to make this. Some reptile companies who carry heat tape will assemble a mat for you according to your specifications, but some people choose to make it themselves. And heat tape gets very hot. You must control this with a rheostat.
For a Do-It-Yourself guide on assembling a mat made from Flexwatt Heat Tape, read this:http://www.arbreptiles.com/cages/flexwire.shtml
Heat tape can be a good choice for energy and cost-conscious pet owners since it is versatile and you have a lot of control over the size, but with anything that is adapted for reptile use, proceed with caution.