XML test documents

I'm currently developing a XML pretty-printer Perl module which i'd like to
test before releasing as Open Source. I wondered if there was a good
collection of XML documents for exercising XML processors which i could use
in addition to my own feeble tests ?

I'm particularly looking for documents which exhibit a good range of DTD,
characters, predefined entities and any common trip-ups which i might have
missed.

TIA
Paul
--
Paul Sumner Downey
http://blog.whatfettle.com