I've said it before and I'll say it again, Liveview and the magnify feature. Tripod mounted of course helps immensely but I have managed hand held and resting on things when using an IS lens. A non-IS lens would almost certainly need a tripod. I've been able to id stuff that would otherwise need a scope with a powerful mag eyepiece or the longer end of a zoom eyepiece. Also is good for just viewing something interesting far away, when I saw the Pied-billed Grebe at Hollingworth Lake it was a long way off and not doing much so I just popped on the Liveview and zoomed in on it and it was like looking through a scope but with one bonus, a lot of other people who turned up to see it were very happily gathered round my camera enjoying the views which many of them wouldn't otherwise have had. I never got any pictures that day after a long journey but all the thanks I got from a lot of people who had just come along with bins more than made up for it.
Downside is obviously the battery consumption but on a 1D battery it didn't make much of a dent. Also tracking a moving bird with it might be a tough proposition. It's a win win as far as I regard it though cos of course requires no additional spending whatsoever! One day I'll devise some kind of experiment to demonstrate just how far away it can work. Ye heathens shall be converted!