Winter tyres: Brilliant in cool to cold conditions. Far more effective on snow than you might imagine.
Chains: necessary in French ski resorts. Effective but can be a PITA to fit and remove and a not fun when they snap. And horrible on the bits of tarmac.
Socks: light, surprisingly effective, easy fit on the roadside and become more effective as they wear. Worth riding them on tarmac a bit when you first get them to make them rough-up a bit. Probably wont last that long but fail without drama.