XML and webservices are some superglue that can connect websites and applications.
This site is full of very carefully choosen XML websites, books, discussion groups.
And since one of the best uses for XML is webservices I place both topics here.
XML is a very specialized tool -- I see it used for the wrong reasons, when other tools are better. I see the wrong XML technology being used where later or different XML standards would work much better and cleaner. I see it used instead of a database as a poor foundation beneath an application "house" that will collapse under the weight of users, the lack of concurrency, or just the absurdity of it all.
So how do I recommend you incorporate XML in your applications, websites, and utilities?
I recommend your programs import and export several XML formats and thes primary inputa nd output is XML, do not store your data in XML within the application coding. And in your import/export routines, clearly define your schemas and grandfather support past schema versions rather than demanding all import/export be in the your XML formats. Import and export not store natively, databases are best and fastest at storing and manipulating data. Within your database text "blob" fields consider XML formatted strings with schemas for validation so those blobs are more structured.