Televisions built since March 1, 2007 are required to have ATSC tuners (the standard for Over the Air broadcasting). So, your newer 'digital' TVs should not need any converter. Read the manual to see how your TV is to be set for reception of OTA (not cable).
http://www.dtv.gov/consumercorner_2.html
Of the standard definition converter I've had experience with, the Zenith DTT901 has been the best performer. Sadly it is in short supply and the price has skyrocketed. If you can find one new for less than $100 you're doing very well. Given the price of the standard definition product, you might as well consider a high definition capable product such as the Channel Master CM-7001.
http://www.channelmasterstore.com/HD..._p/cm-7001.htm