An X11 web browser, created by W3C and also developed by Yggdrasil.
Arena was W3C's reference ("testbed") web browser. The problem with it was that it couldn't do well on pages that were not strictly standards-compliant (and many pages that day were not, and regrettably still aren't).
Yggdrasil's marketing hype (from yggdrasil.com) sounds rather interesting:
Arena is a graphical web browser consisting entirely of free software. Its origins predate proprietary packages such as Netscape Navigator, Microsoft Internet Explorer and Mosaic. It is the source of a number of innovations which have since been copied by other web browsers, such as HTML tables and style sheets.
...and it even had a limited CSS support!
It had a light brown interface, with brown marble background default - and the links were not underlined, they had a cool 3D "button" look (different from form buttons, of course).
The coolest innovation in the UI was, I think, that it showed on browser toolbar if the document had invalid HTML in it. Such warning things would make Big Browsers perrrrrfect and would probably make "increased standards compliance of the Web" reality. =)
Arena was abandoned by W3C, and their current "testbed" browser/WYSIWYG editor is called Amaya (that also does better rendering job and won't crash as often, but it's still more useful as a web editor, not much as a browser).