Here's a list of 747 operators and the type, serial # of their aircraft:
http://www.airfleets.net/exploit/production-b747.htm
Most of the major airline passenger services are -400's but there are some of every model still in service somewhere. There seem to be few 747SP still in service one with Saudia, in executive configuration I think, and SN 21758/371 that was delivered to iranAir in 1979 and is still in use, as well as three other even older SP's. In one of the more bizarre incidents of my life I signed the loan agreement for the engines and airframe in Tehran in June-1979. if I recall, during the brief Bazargan government (Feb-Nov 1979). The loan commitment had been made and even though the Shah fell the delivery had to be made. I flew in from Dubai, signed, and left the next day. I also flew on that airplane.
There are a handful of -100 still in service if you want the original 747. They are with Saudia Arabian airlines, five of them, Kabo Air of Nigeria has three, and Orient Thai has one.
The US scheduled carriers United has 25 and Delta has 16. All of those are -400's, the newest version. Of course you can choose from most long distance European and Asian airlines for 747-400's, some of which are very new. To fly the newest one, sn 1358, go to China Airlines which took de3livery in April 2005. The later 747's delivered were freighters.
if you wait a year or two you can fly in the new 747-800, a stretched version, for which Lufthansa is the launch customer, making them the only airline to have both the 747-800 and the A380.