Windows XP SP3 (Service Pack 3) is available now as an optional update on Windows Update and will become automatic (probably as a critical update) on June 10th.
If you are using Microsoft Dynamics, Microsoft's POS (point-of-sale, not the other thing you're thinking) retail solution, don't install SP3.
Microsoft requires you to have SP1 installed before SP3, and recommends you have SP2 installed (get SP2 from Windows Update) as well before SP3.
In a nutshell, if you have been running updates all along, this won't be a big deal. However, if you load Windows XP fresh on your PC for some reason, this will save you a great deal of time. It adds some Windows Server 2008 compatibility and other networking features, but the big deal is saving time after installing SP2.
So, don't expect any new features. Getting SP2 was essentially getting a new operating system. SP3 is just a time-saving roll-up of patches.
You can download Windows XP SP3 here (316mb) or (more official looking) here. Of course, you should find it as an optional update.
For more, Paul Thurrott's got a review.
Yeah, I know, lots of boring technical stuff lately, but it's what I do for a living.
Update: Microsoft has released a hotfix for Microsoft Dynamics.