In the 1964 edition of the DOD/AEC book The Effects of Nuclear Weapons, a section titled Radiological Warfare details some of the most common WMDs. The fission products from a conventional nuclear explosive weapon are as much a radiological weapon as weapons solely designed for the purpose of mass radiological warfare. The standard high-fission thermonuclear weapon is automatically a weapon of radiological warfare, as dirty as a cobalt bomb.
Initially, gamma radiation from the fission products of an equivalent size fission-fusion-fission bomb are much more intense than Co-60: 15,000 times more intense at 1 hour; 35 times more intense at 1 week; 5 times more intense at 1 month; and about equal at 6 months. Thereafter fission drops off rapidly so that Co-60 fallout is 8 times more intense than fission at 1 year and 150 times more intense at 5 years. The very long-lived isotopes produced by fission would overtake the 60Co again after about 75 years. Other salted bomb variants that don't use cobalt have also been theorized.
A far lower-tech radiological weapon than those discussed above is a "dirty bomb" / radiological dispersal device, which refers to a conventional explosive bomb with a radiological side effect due to strapping radiation sources to it, is a very inefficient way to spread radiation, and all such "weapons" have problems that render them likely impractical for military uses.
Rather, radiological warfare with dirty bombs would be of vastly more use to terrorists spreading or intensifying fear, uncertainty and doubt. The release of radioactive material may involve no special "weapon" and include no direct killing of people from its radiation source, but rather could make whole areas or structures unusable or unfavorable for the support of human life. The elevated radiation levels in the targeted areas would make these areas dangerous to humans. An area, once contaminated with radiation, is often expensive to clean up. Decontamination of the built environment would take time.
Like land mines, radiological weapons can be used as an area denial weapons method.