The Canon EF-S 17-55 F2.8 is the best "walk around" lens for this camera - I have one for my 7D.
Your 18-200 covers a broad shooting range (Including that of the 17-55). The best way to complement it, IMO is with wide aperture primes. The 50mm is a good starter. Get that and compare the photos you can take at F1.8 to those that your 18-200 produces.
If you are going on a safari or shooting sports from the sideline, then maybe something w/ reach over 300mm is good.
Good site to learn:
http://www.michaelthementor.com/
Edit to add: The 18-200 is perfect for a round the world trip.