I finally tried out Opera 5.0 for my aging original iMac. It works great. Except on Mac Rumors. it won't let me login. whenever I type in my username and password, it says it was "invalid"
I've mainly been using Netscape 7 for forum browsing when on my 8.6 systems. You could try Opera 6... but it takes some fiddling with the Carbon libraries to get it running right as I recall (I think you need version 1.3 or higher of the Carbon Lib to run it).