Depends on the plant - I have a houseplant (Schefflera) which is supposed to be very difficult to propagate, but I do it all the time by sticking the tops in water,works really well. You can always try it with any plant, but it won't always work.

Mint roots easily in water too. It's always worth a try.

I always propogate my busy lizzies in water they take really easily. Use them as a houseplant in winter then pop them out in the garden for summer.