I'm going to second the "shoot with what you have for a while, first" argument, particularly if you're new(ish) to photography. What do you like to shoot? Landscapes? People? Macro? What area of your zoom do you tend to stay (18? 55? Somewhere in between?)?
Once you've figured all that out, you'll be in a better spot to figure out which second lens to get. If you're looking for more distance, go with a bigger zoom. If you want better close-up (and/or low-light) capabilities, then go for a fast Prime. A Prime lens will probably cost you more than a zoom, but it's well worth it (IMNSHO).
I bought my camera with 2 kit lenses - a 14-45mm and a 40-150mm lens; I shot with the 14-45 for a while, and realized I do mostly close-up work at the 45mm end of the lens. I then bought a set of close-up filters to play with, and realized I love Macro work, so I broke down and bought my 105mm Macro lens.
It almost never leaves my camera.
Time from camera purchase to Macro lens purchase: 10 months. No sense in buying extra glass I'm not going to end up using. For that matter, I'm probably going to (try to) sell the 40-150mm lens, because I purchased a 70-300mm lens for the extra range (and even that, I seldom use).