xml-emitter
xml-emitter simply emits XML, with some complexity for handling indentation. It can be used to produce all sorts of useful XML output; it has an RSS 2.0 emitter built in.
$ area51 add xml-emitterCopyUsage
;; area51 add updates your .asd automatically.
;; Use the package in your code:
(in-package :my-project)
(xml-emitter:some-function ...)