The Raynox DCR-250 gets my vote as an excellent add on for all types of cameras - P&S and DSLR... I was just waiting to give someone else a chance to be the first to mention it
For magnification ratios, you are talking about how large the photographed scene is compared to the sensor in your camera. In my D40, the sensor measures 23.7mm x 15.5mm (looked up on
dpreview.com). Taking a close-up shot of a ruler, I could get as close as about 42mm wide x 28mm high (at 55mm on the kit lens and using the DCR-250). That means I'm getting a little closer than 1:2 but not down to "true macro" at 1:1.
To get down to 1:1, I think i would either need a bigger sensor or better magnification (so that I could view about 24mm of the ruler across the frame).
Wulf