Just stumbled across this. I had never heard of it.

If you are just going from A-B, then possibly not for you, but for someone wanting to see the country, sample the complimentary food (and drink) on Virgin or East Coast, enjoy the delights of a sleeper service between London & Cornwall or London & Scotland, whilst exploring the country when your at it, then this ticket may just be for you.

Www.nationalrail.co.uk/times_fares/pr20780b0a0400020167ed620a7e504e.aspx

It isn't possible to use it on the Metro, DLR or underground - but still excellent value, especially with the discounts available if your a railcard holder.

I've held the friends and family one for a few years now and had my moneys worth back, several times.
If you didn't have a railcard, it would be worth investing in one, just to get the massive discounts on this.

edit: Just for info, here is the entire list of rovers available to purchase - www.nationalrail.co.uk/times_fares/rangers_and_rovers.aspx