Various developers have created implementations and test suites for the URI specifications, and have discussed them on the uri and related MailingLists:
GrahamKlyne: UriTest.n3; also .rdf, .xls, .csv. see discussion 8 Feb 2006. An updated version of these test cases is available in Haskell source code at URITest.hs.
Sam Ruby: Urlnorm
MSM URI spec impl in XSLT (somewhat like uri.xsl by DanConnolly)
TimBL and DanConnolly swap/uripath.py contains unit tests
Martin Dürst: uri-rel-test.html
DavidPowell: The above tests in XML format, with an XSLT test-harness for an XSLT implementation of a URI resolver
MikeBrown: the unit tests for 4Suite XML's Uri.py (for Python) contain many of the above, plus a few more from various sources. this page runs many of the same tests against the reference resolver in his uri_funcs.js as well as TimBL's uri.js.
DaveBeckett: Raptor C libraries for RDF contain URI parsing and resolving code, but no tests
Sebastian Pipping: the uriparser library (written in C) contains a number of test cases.
Aaron Swartz: an URI test suite for testing a tcl parser.
Jeremy Carroll: Towards a test suite ... 21 Dec 2004
Martin Dürst: Internationalized Resource Identifiers (IRI) Test Overview work in progress around Apr 2005
Björn Höhrmann: javascript-based interactive grammar checking against RFC 2396 and RFC 3986, but no test suites
swap/uripath.py and Uri.py also contain functions for deriving a relative URI reference from an origin URI and target URI.
(LinkMe: notes from python conference where DanConnolly talked with Guido about urlparse.py in the python std lib...)
Nearby... UriSchemes, W3C URI Interest Group.
earlier: URI spec review: URI specification (RFC 2396 revision), some review notes 29May