XOXO (eXtensible Open XHTML Outlines) is an XML microformat for outlines built on top of XHTML. Developed by several authors as an attempt to reuse XHTML building blocks instead of inventing unnecessary new XML elements/attributes, XOXO is based on existing conventions for publishing outlines, lists, and blogrolls on the Web. The XOXO specification defines an outline as a hierarchical, ordered list of arbitrary elements. The specification is fairly open which makes it suitable for many types of list data. E.g. the more semantic version of the S5 presentation file format is based upon XOXO.
Flying Saucer is an XHTML renderer written in Java. It's 100% Java, not a native wrapper, and it only handles well-formed XHTML + CSS. It is intended for embedding web-based user interfaces into Java applications (ex. web photo album generator, help viewer, iTunes Music Store clone). It cannot be used as a general purpose web browser since it does not support the malformed legacy HTML found on the web, though recent work on compatibility libraries may be enough to do what you need. You may be able to work with legacy HTML (e.g. HTML that is not well-formed XML) by using a pre-processor that cleans it up; there are several of these, including JTidy and TagSoup.
This service tunnels HTTP requests and changes the MIME type of text/html resources to application/xml or application/xhtml+xml. This allows accurate testing with XHTML user agents such as Firefox, Opera or Amaya.
This service lets you validate XML documents such as XHTML against the appropriate schemas. It performs a more accurate validation than the W3C validator.