I use firefox and found many addons that I find very useful. Exif viewer FxIF is the one I use mainly. I know there are ones for changing colour spaces but I tend to keep my browser in sRGB so don't use anymore since pretty much all stuff on web is sRGB and anything I upload is too. It's no good me seeing it correctly and everyone else getting an incorrect version, especially since I don't use browser for local viewing. Still might be useful to some. Also a half related thing is cooliris, it's just a fancy eye candy style viewer (for win version only I think) for arranging walls of thumbs etc. I got it a while ago but never tried but I keep meaning too as it looks interesting and nice even if functionality wise it's pointless.
Way off topic and unrelated (but might be usueful) the other addons I use (from occasional to can't live without) are no script, greasemonkey, stylish, foxlingo (very handy), firebug, fireftp, mediaplayer connectivity, adblock plus, pdf download, foxy tunes, ie tab (win only), and a few others I don't really use much now like ipa and whois tools. I end up disabling the unused stuff (resources etc) but keep in case ever need once in blue moon.
Any others photo related especially but any recommendations welcome that people use?