This is more a comment than anything actually useful, but if you want to test for a REALLY obscure browser, how about the in game browser from EVE online?
I have never even played the game, but I know somebody who's quite into it, and it has a browser within the game that is intended to be used for sort of special game-centric websites. However, the browser will attempt to render other pages, so hypothetically you could actually browse the web from inside the game.
Of course, since it uses a very crude custom engine, it frankly sucks horribly--although it attempts to render CSS, it has no float support, so looks worse than it would if it just ignored stylesheets entirely. I am not so insane that I would actually try to code for it (though I admit if there were an easy hack for hiding stylesheets from it, I'd do it), but I have done second-hand testing to see that it's at least roughly useable.
My testing protocol involves making things look perfect or nearly so in Safari 2+, Firefox 1.5+, Camino (because that's what I use), and Opera. I make sure it looks at least good in IE7 and IE6 (I installed a standalone version of the IE7 beta and left six as the default on my Parallels windows install, to avoid the issues), and that it's at least passible in IE5 and 5.5 (also standalone installs).
I also test in IE4 to make sure the site is at least useable even if it's ugly, and I hide the styles entirely from NS4 with "media: all" so I don't really need to do more than keep my document structure logical (I was so glad to see NS4 drop below 1% so I could start ignoring anything but basic functionality in it). That also covers Lynx or other text-only browsers, as well as IE3 and NS3--no stylesheet support. I haven't put a lot of work into screen readers, although I do try to keep the structure efficient.
I'll also use one of those online tools to at least run things through Konqueror and earlier versions of Safari, but most other browsers are standards-compliant enough that it's not an issue.
What I haven't been able to test with yet is WebTV and derivatives (there was a nifty WebTV browser simulator for OS9, but no love on a MBP), the PSP browser (don't have one), or mobile phone browsers until I saw that neat Opera Mobile tool posted here. There was also the Dreamcast browser--I knew somebody who used to use that, but that was a long time ago.
Incidentally, although the numbers are vanishingly small, I do still see occasional hits in my logs for Opera 7, Firefox "0", Safari 1.0, and even Netscape 3, IE3, and IE *2*, if you can believe that. I suppose it could be somebody goofing around with browser ID strings, but there are some pretty darned crusty computers floating around out there. Amusingly, NS6 actually gets LESS hits than NS3.