One of the most solid open projects is, in my opinion, the entire Mozilla family (I include mozilla, firefox, thunderbird and all the others) which can be found here.
I have been a happy and satisfied user of mozilla’s suite, although lately, I have found that firefox and thunderbird cover my needs better. However, the past 48 hours I am experimenting with swiftfox, a lite version (could we say this?) of firefox. Fast, agile, doesn’t get stuck. The perception the user gets is pure firefox, but it is a faster deal than the rest. Yes, for the purists, it does not render well the acid2 test (but with the exception of Konqueror, I don’t know a browser that does).
The version in the image above is the latest 22.214.171.124 for amd64 processors. It came with all the bell and whistles (thanks to automatix). That meant that I had java, and mplayer and flash, which I could not have before without having a firefox for 32 and a regular firefox for 64.
Bottom line, I like it and I will keep on using it.