WSS and W2K3 R2
In W2K3 R2 there are a number of updates to WSS. They fix some of the biggest deployment issues customers currently face:
Support for IP-bound virtual servers
Support for advanced extranet configurations
Windows SharePoint Services running on ASP.NET 2.0 (Whidbey)
Windows SharePoint Services support for Windows x64 editions
Check out a description of each here:
The best on in my view is the "advanced extranet configurations" as it causes plenty of pain in a couple of my customers.