Great Seats, Inc. A Washington, DC based ticket brokerage. They generally have very good seats for shows, particularly in the DC/VA/MD area. They also have some for Boston, Charlotte, and Atlanta
Tixs.com This is a classified service where you can find tickets for sale or post an ad about wanting tickets. There are both ticket brokers and private sellers listed here. This is a pretty good service, I have used it before to sell tickets.
INTERTIXX This site has a lot of listings for the New York, Cincinatti, and Wisconsin shows
EbayThis is the online auction house. There are several tickets listed on here from shows around the country. There are 10th row seats on here for the Michigan show, and right now the highest bid is around $800!
Ticket MagicThis brokerage serves the New England area and is located in Massachusetts
TNT TicketsThis is a huge company, dealing with up-front seats all over the country...but pricey
Executive Tickets serves Ohio and surrounding states
Ticket Inlet Another Washington, DC based brokerage
Yahoo Auctions As of 8/19, Yahoo had quit a few tickets on auction, and the prices were fairly reasonable...several had no reserve price