A source I normally find reliable offers the following list of current operators at 1 May. Sure, not all of them fly every day.
http://members.chello.nl/s.c.verbrugge/types.html#dc-8
40 United Parcel Svc-UPS
19 BAX/ATI-Air Transport Int
8 Arrow Air (N/HP)
8 Astar Air Cargo
7 Airborne Exp
3 African Int
3 BETA Cargo (PR)
3 Johnsons Air (9G)
3 Murray Avtn
3 Skymaster AL (PP)
2 Cargoplus (9G/N)
2 Cygnus Air
2 Heavylift Cargo (A6)
2 Hewa Bora AW
2 Kinshasa AW (9Q)
2 Silverback Cargo Freighter (9XR)
1 Aeropostal (XA)
1 Al Nassar VIP
1 Al-Dawood Air (5N)
1 Sigair (VIP) VIP
1 Congo Air Force (9T)
1 Espace Aviation (9Q)
1 Expo Cargo (4R)
1 Gabon Gvmt VIP
1 MAS Air (XA)
1 MK AL-Cargo d'or(9G)
1 NASA
1 Peruvian Air Force VIP
1 TAMPA
1 TCB (PR)
1 Togo Republic (5V) VIP
1 Trans Air Cargo (9Q)
1 United Arabian AL (ST)
126 in service
243 scrapped
109 stored
78 W/O
556 built