I'm assuming you have a Canon, using the 18-55mm kit lens, right?
Correct. I am using the Canon Rebel 350D/XT with the kit 18-55 lens, and am looking for a good upgrade...I have read Gordon's upgrade reviews, but I want something with a little more zoom, I am looking at the Canon 28-135 IS lens. I read somewhere on this forum that it got just OK reviews though. Are there any similar non Canon lenses with IS? I also heard about a Sigma lens coming out with something like 18-200 with IS...
It depends on how much you are ready to spend for an upgrade.
If you want a very strong upgrade, then you are best to look at the L line. Things like the Canon EF 70-200mm f/4.0L USM, Canon EF 70-200mm f/2.8L IS USM. Maybe you want to get into macro, then a Canon EF 100mm f/2.8 USM Macro lens would be great.
It really depends on what you plan to use it for and how much it matters/you want/can spend.
Check eBay or Craigslist (if you are familiar) for good deals on lenses and equipment.