There are a lot of solutions and it depends how cold and also on your riding style.

People here are saying ride flat pedals instead of cleated pedals. yeah, that works, but a good compromise in that direction is something like these


that way, you can wear your warm show or hiking boot and still get some of the advantages of clipping in.

now i have a great pair of winter boots with spd cleats -Bontrager OMWs. They rock. Much lighter and less expensive than 45Norths or Lakes, but possibly warmer. But for some reason they didn't sell and were discontinued.

More general comments - Sock liners! Thin Smartwool sock liners with thicker wool blend oversocks.

And chemical toe warmers.

Also, if you are wearing normal cycling shoes (because your conditions are cool and maybe wet and windy, but not seriously cold), then you want to think about good overshoe protection. Forget those lousy Louis Garneau or Pearl Izumi gore-wear things. Go for the neoprene. i like the Enduras, but there are other brands.