I've used a reptile mat in the house (in an enclosed box) to start seeds off, but my understanding is that the elements in them break easily (if there is a point-load on them), so a horticultural warming mat would probably be better.
I don't bother with any of that any more, I just stick them in our boiler room, until they germinate, but that room is toasty-warm so not an option for everyone :)
Airing cupboard perhaps? Top of fridge?
