NOTE: This is a repost of an entry that I wrote for the molecularecologist.com.
You may call them plate sealers, we call them "sticky-tops" (as opposed to the rubber plate sealers we use for PCR). Whatever you call them, they seal up 96- and 384-well plates so you can store them in the fridge or freezer. Many vendors sell these handy sealers at a pretty high cost (to you), particularly given that you are basically just paying for tape of various sorts. Anyway, this brings me to my point...
Outside of using a heat-sealing system, I really like aluminum plate sealers, as I think they seal much better than their counterparts (the terrible clear sealers). Sadly, the aluminum plate sealers sold by vendors are quite expensive. Because I dislike being ripped off, we use >3M Aluminum Foil Tape 425 Silver - 3 in place of vendor supplied plate sealers.
The 3M tape (make sure it's the 3" variety) looks something like:
A single roll retails for about $79.00 for 60 yards of material (you can find better prices, depending, just google it). That is equal to 2,160 inches of tape, and if we assume you use a generous 4" of tape to seal each plate, one roll should give you 540 sticky-tops. This works out to about $0.14 per top. Generally speaking, if you get a good deal, you pay about $100 per 100 vendor-supplied plate sealers ($1.00 per top), so the 3M tape represents about an 85% savings. In addition to the tape, you'll need a means of cutting it from the roll (we use a box-cutter/carpet knife), you need to be careful not to cut your fingers on the edges of the tape, and you need to press the edges against the plates to seal them well - this tape is thicker than that supplied by most vendors.
*Disclaimer - we do not use these in our thermal cycler, choosing instead to use rubber sealing mats, which we wash in 10% bleach and rinse 3X in DI between uses. We find that the mats really minimize evaporation relative to other sealing mechanisms. So, I can't speak to the efficiency of the 3M tape for PCR applications - largely we use it as a storage aid (sometimes storing sealed plates at -20 C for months). YMMV.