Without much notice, publicity, hue and cry, Microsoft released Service Pack 2 for Windows 2003. This release introduces significant collection of patches and a couple of new features to the operating system.
From experience, upgrading SP1 of Windows 2003 a while back broke a lot of things including 3rd part applications, so, make you do your homeworks and test in non-production environments before deploying SP2. Please also disable Automatic Updates for SP2 if you wish to do testing first and if Automatic Updates are turned on in your production machines. There's a toolkit available to do this here.
W2K3 SP2: http://www.microsoft.com/technet/windowsserver/sp2.mspx.