SeatGeek is a ticket aggregator (think Orbitz for tickets) that shows you all of the tickets available from other ticket resellers. SeatGeek tends to have the best deals, and they have transparent pricing, so you don't have to worry about checkout fees.
StubHub, owned by eBay, is the most prominent peer to peer reseller but they add significant fees when you check out so beware. Stubhub has the most inventory of any reseller, but it doesn't always have the best deals because of its fees.
Ticketmaster is a trustworthy and reliable seller but the selection of tickets doesn't tend to be quite as good as some of the other ticket websites.
Similar to RazorGator, Fanxchange is also listed on SeatGeek's marketplace so you don't need to shop directly on Fanxchange's website.
TicketZoom is a low traffic / volume marketplace compared to the other sites listed on this page so it's highly unlikely that you'll find a better deal on here than on StubHub, SeatGeek or one of the other listed sites.
Your best bet for the lowest prices and largest selection of tickets is SeatGeek.