One of the most useful and cost effective items I have come across using my new camera is an attachment, a lens really, that attaches to the camera and sits then to your regular lens. I found an auto focus teleconverter for my Sony alpha that increases the range of my lenses to twice there normal range. So, if I use it between the camera body and my 28-70 lens, my range is increased to be 56 to 140. Sony makes one of these lenses, and when I priced it, it was a bit out of my price range. Shopping on Amazon, however, I found a Bower 2x auto focus teleconverter for Minolta which works on both my Minolta film cameras and my Sony DSLR. I must admit, though, the fit is a bit tight. But, for the days when I do not want to carry around all my lenses, taking the teleconverter (which is small and very portable - see above) and a single lens, gives me the flexibility I need.
Update - I was a little troubled the other night when I tried to use this adapter with the 18-70 Sony lens that came with my Sony alpha. I was unabel to clearly focus using the auto-focus, and have yet to determine why. Hmmmmm. . . .