I forgot to add FireFTP and British English Dictionary!!
+1 to both of those and Adblock, plus
DePhormation (dodge that nastiness !)
CustomizeGoogle (tho' Google seems to be fighting back
)
Better Gmail
User Agent Switcher (for those irritating sites that complain that you have a "non-standard (ie Non-MS) browser")
Met Office Weather Gadget
and for the slightly more geeky:
Greasemonkey
Firebug
DOM Inspector