Compost

Stick them in the compost heap or on the soil as soil conditioner - although one main caveat is that if you grew tomatoes in them, don't put them where you plan to grow spuds or tomatoes next year (rotation).

Or something I've done with my large pots is to reuse old compost at the bottom of them and fresh compost at the top - but not if there were any diseases in the old compost, of course