When viewing a webpage in Camino, hold down the Command key, and then scroll down to increase font size, or scroll up to decrease font size. This allows a more accurate font decrease/increase then Command-Plus or Command-Minus.
I don't know about all the other browsers, but I've tried this in Safari, and it doesn't work. I could not find any Camino documentation on this functionality.
[robg adds: This also works in the recently-released Firefox 1.5.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051201120732892