Mine is an Ashford 16" rigid heddle. As Sally says, they are extremely simple and quick to warp up.
The other popular make seems to be the Kromski. It has a kind of warping board underneath, which, if you want to use one, can save some money.
The Ashford Knitters' Looms and the Kromski Harp looms fold and come ready lacquered.
The Ashford Rigid Heddle does not fold and the wood is unfinished. (And these looms are therefore cheaper.)
The Ashford looms all have notches under the rear supports, so you can use them on your lap with the end resting on a table.
Some people love their Kromskis, but I have to say that I have seen several people (including my sister) buy and then sell Kromskis, and then buy and keep Ashfords
. So it would seem that the Kromski may be a bit Marmite, whereas most people seem to get on with the Ashfords.