When you need to stand a ladder on uneven ground, you always have to mess around to get something sturdy under the feet. Now there is an incredible invention from British Columbia that attaches to the bottom of any ladder.
Place it on uneven ground. Step on one of the two release levers and the other foot slides down to solid ground. Release the lever and you are ready to climb the ladder. It can be added to wooden, aluminum or fiberglass ladders. As a bonus it spreads the footing out a bit for even more stability than your original ladder. It is not cheap, but neither is a fall from an unstable ladder. Every time I start using mine in front of any tradespeople, they immediately want to buy one. I would place this as one of the best products I have run into in years.
It is unfortunate but this easy to adjust and quite secure model is not always easy to find in stores and in fact the inventor has stopped producing new stock while he is re-engineering it to bring the price down -- in an effort to get wider store distribution (October 2011). Update January 2013 -- it is back on the market with the name BaseMate Ladder Leveller through the US industrial distributor GuardianFall.com. This new model clamps into the first two rungs, adding an extra step to the bottom of the ladder but not modifying, cutting or bolting through the ladder, which would nullify any original warranty on your ladder.
You may find other models where one foot or the other adjust and they are certainly better than sticking a brick under one side of a ladder. This is the only one I know that without any tools, or even having to bend over, has an infinite adjustment, even a built in sprit level -- encouraging you to set it perfectly vertical even if you only move over a foot. Being able to easily move your ladder and get it vertical again is a real safety factor -- you are less inclined to lean out too far and tip everything over.
As I look at the stores in 2013, I find very few quality accessories for extension ladders. This is really a neglected line as many good accessories like the Basemate do exist. Follow this link to my article on Speciaity Ladders including an indoor leveller for painting in stairwells.