In your case, for free, none. You are at the highest dot revision for Mac OS X 10.3, "Panther."
If you purchased Mac OS X 10.4 (from many used or new retailers), the next 3 chronological upgrades would be 10.4, 10.4.1 and 10.4.2.
The 3 next chronological "point" updates (major, paid-for upgrades) are 10.4,10.5 and the unreleased 10.6.
You can skip a major revision, going directly from 10.3.x to 10.5.x. You do not have to install the in-between upgrade in that process (the rules are slightly different if you elect to do an "Upgrade Install" over an "Archive & Install" or "Clean Install").
10.4 you can find through many Mac retailers (with the exception of Apple themselves). 10.5 is currently being sold by Apple, and is the most current Mac OS X release. 10.6 is right around the corner but is yet unreleased (should be sometime in 2009).