If you think XML is full of crap that no one needs (DTDs, anyone?) or could have been made simpler (we need element names in end-tags why?), then Paul "PaulT" Tchistopolskii's page of XML Alternatives is for you. He lists a number of projects whose goals are similar to XML (to structure data for computer use). The list also includes things like intuitive text markup schemes (like wiki text), and front-end languages for bizarre XML syntax (like NiceXSL).
Of course, none of these alternatives will come anywhere near XML's ubiquity, but it's good to know there are other possibilities. Lots of people use XML in situations where interchange isn't required, and other options are perfectly reasonable choices.