If you're looking for an actual scope mount that will do that, the LaRue is very nice...I just got mine today. Unfortunately, it's expensive...$215.00 or so. It's very high quality, and looks like it will last for a good long time. The thumb levers are adjustable, which is a big plus. This will extend the scope about 3 or 4" or so, and the rings are built in.
If you already have rings, and just need some extension, then look at the ARMS #36....that could save you some money, if you can pick one up cheap in the EE.
I have no experience with the YHM thingie, so I can't really comment on it.
Out of curiosity, what scope do you have, and how much futher do you need to go. IIRC, the Armalite will push the scope forward a *little*, but not much.
The RRA is a huge mount, and it doesn't really get you anything in terms of moving the scope forward.