Personally I use the Brookstone Back Stretcher & Ab Trainer.
It's more stable than an exercise ball and really works your abdominals which in turn takes weight off your back... though I use the exercise ball for traveling.
Swimming has to be the best exercise because flotation takes a lot of weight/stress off your back... mind you I'm not talking about 100m backstroke.
Sadly my new apartment complex doesn't have an indoor pool for winter
Also I found that I can ski as long as I avoid moguls and crashing... usually my legs will tire before my back does and again you don't have the impact of walking or running. And I get to sit between runs on the chair lift and enjoy the scenery.
Good endorphins from the snow covered mountains, as anyone watching the Olympics may have experienced (on a lesser degree than the actuality of seeing the beauty of the mountains in person). Being able to adjust your perspective and take as much time and what ever you decide interests you, instead of the cameramen, breathing in the fresh air away from all the city smog... the whole experience.. Great Endorphins!
Elliptical machines are also good if you want the aerobics of walking without the impact, but hard if standing for long periods increases your pain.
You can also look into recumbent bicycle exercisers. Basically you are laying back and pedaling. Now pedaling will impact your lower back as your quadriceps origin at your lower back, but for me cycling is helping. I just have to pace myself and start off slow and short and work my way up to longer times... but if you think about it pedaling is very similar to knee to chest stretches...