Depends on your location. For example, in my area (north of Scotland), I monitor all the following frequencies:
Scot Mil ATC C/S 251.4750
Scot Mil ATC E 278.5500
Scot Mil ATC E/S 251.2500
Scot Mil ATC E/S 371.8750
Scot Mil ATC W 233.3250
Scot Mil ICF 134.3000
Scot Mil N STC 281.4000
NATO Approach 123.3000
NATO Approach 362.3000
NATO Radar 385.4000
NATO Tower 122.1000
NATO Tower 257.8000
Kinloss Ground 389.4000
Kinloss Maritime 389.5000
Kinloss PAR 118.3500
Kinloss PAR 240.4750
Kinloss PAR 279.1500
Kinloss Tower 235.0250
Kinloss Wing Ops 278.9750
Lossie 12 Sqn AA 234.0500
Lossie 12 Sqn AA 240.3750
Lossie 12 Sqn Op 360.4500
Lossie 14 Sqn AA 293.7750
Lossie 14 Sqn AA 375.4500
Lossie 14 Sqn Op 241.5000
Lossie 15 Sqn AA 278.5250
Lossie 15 Sqn AA 311.8250
Lossie 15 Sqn AA 363.6500
Lossie 15 Sqn Op 386.8250
Lossie 617 SqnAA 294.9000
Lossie 617 SqnAA 366.5500
Lossie 617 SqnOp 339.8500
Lossie Approach 234.8750
Lossie Approach 277.8750
Lossie DATIS 264.7750
Lossie Ground 268.6250
Lossie PAR 244.3750
Lossie PAR 369.2000
Lossie Radar 118.9000
Lossie Radar Dep 119.3500
Lossie Radar Dep 308.8500
Lossie Radar Dir 258.7000
Lossie Tower 118.2000
Lossie Tower 279.0500
Lossie Wing Ops 369.3000
Tain Range 122.7500
Tain Range 279.1250
Tain Range 339.8000
Lots of military frequencies listed in the book Airwaves 2008.