I haven't come across any more functional browsers than the ones that you mentioned.
The problem is that many sites are aimed at supporting quite a high feature set from either the Gecko (Mozilla) or the Trident (Internet Explorer) browsers.
There are few other commonly used layout engines, obviously Trident layout engine is a M$ proprietary blob that nobody wants to use for anything other than 'toy machines' and Gecko is a little big.
The other two layout engines that do get some support are KHTML from KDE (used by Konqueror etc.) and GTE-WebKit which is derived from Apple WebCore which in turn was derived from KHTML
Few browsers are available based upon the GTE WebKit at the moment and there are some portability issues over the GTE WebKit due to some use of non portable Posix extensions. It was suggested that GPE-Minibrowser, which is based upon WebKit, could be a good candidate browser for OpenBSD on the Zaurus.. in practice it doesn't seem to offer much more functionality than Minimo (which is useable) and has a few dependencies outside of the ports tree.
I check
http://en.wikipedia.org/wiki/List_of_web_browsers from time to time to see if something new and interesting pops up.
EDIT: Sorry, forgot to mention the Presto layout engine as used by Opera - Opera isn't open source... it is available linked for Qtopia on ARM but it seems nothing else on ARM... and I'm sure you don't want to ruin your uptime figures by rebooting to launch a browser all the time
-Andy