First, there's Half-off Depot, which I've written about before now. They feature numerous Knoxville businesses and restaurants offering certificates that will cost you only 1/2 their value. The best way to take advantage of a 1/2 off deal is to purchase one for a store that you already planned on using or could use for a gift or special occasion. I can't stress the prior statement more: just because it's half off doesn't mean you should buy it. Sure, it requires some planning and some would rather not be bothered, but I believe planning your finances is the way to financial happiness (The Simple Dollar).
For a good use recently, my husband and I purchased a $20 certificate for only $10, then used it to eat at half price at one of our favorite spots, Sitar Indian Restaurant, for our anniversary.
Second, there's Restaurant.com, which has similar deals, but frankly, I haven't used them yet because they only have limited offerings.
Third, there's halfoffthursdays, which features new offers on Thursdays. I check for upcoming offers and mark my calendar, since some of them go very fast.
Fourth, there's local radio stations, such as WNOX-FM offering certificates. On WNOX, the offers begin on Fridays at 9 a.m. Other radio stations offer deals at other times--Stir-fry cafe goes on sale tomorrow.
These are all I can think of for now, but it seems like there's more--let me know if you know of any more.