It would be great if you could have a widget specifically for downloading a demo of the game, especially for commercial titles.  The "buy now" thing is useful but only really to those already intending to buy, it's possibly even a little off-putting for people who want to try it first and may not realise that there's a demo available - obviously that's down to us devs to advertise ourselves, but a specific widget to encourage downloading the demo would be a nice tool to have.

I would like this feature too - right now I have to write a long explanation on my website to explain how to get to the free demo.

However, if you put the executable of the demo first in your list of files then it will show free or zero price which will achieve what you want - but then beware if you put that on sale and offer a bundle price, the price of that game is calculated at zero when bundle pricing is set.