I use:

GMarks
Google notebook
Location Navigator - for browing URLS in the fashion the page designer didn't want me to
Firebug - debugging web dev and reading pages POST variables mostly
Download Statusbar - because default FF download window sucks
FireFTP - to save running external FTP app
MultiSidebar - not sure how you can live without this one
SnapLinks - open lots of links at once
Tab Kit - can sort pages on 'loaded first'
Greasemonkey - with various userscripts.org stuff in it
qtl - on-demand translator
Table Tools - can live without this but useful for copying html tables in certain ways
BatchDownload
Signature - for pasting repetitive responses in emails

I used to use no addons, but I'm a self-confessed addon junky now. Just wondering what else is good out there.

Adblock Plus
Chatzilla
greasemonkey
Better Gmail 2
Netusage meter
delicious

there are so many cool ones granted and that's before getting to the themes!

australian dictionary is about the only one I routinely install into new installs

I've mucked with a couple like the nagios checker, cooliris or whatever it was called before, and tamperdata but usually can't be assed with plugins cos I use a lot of computers and hate coming to rely on too many things that aren't generally standard

FoxyProxy - allows me to specify pattern-matching of certain proxies to use (eg, http://192.168.0.7/foo/ might go through Service Capture, http://*pr0n.com/* might go through non-work.proxy :)
ColoredTabs - just gives different tabs a bit more contrast
TabMixPlus - small tweaks to tabs, like giving them a constant size rather than resizing when there's more/less tabs open
Foxmarks - bookmark syncing
Firebug - for dev and debugging
Greasemonkey - for viewing nuked Ausgamers posts :D

Netusage meter

Sweet. Added it already :-) Nice find, that is really awesome. f*** having to login all the time to check your usage.

nah man that one is the shizzle, built by australians for s***ty australian caps.

Adblock Plus
Netusage meter
Smooth Wheel
Cooliris
Unlinker (converts image/youtube hyperlinks into the actual element on the page instead of text in a right click menu option)

I normally install DownloadHelper by default for grabbing embedded flv's like youtube stuff. I try to keep the addon's to a minimum though because some can be prone to memory leaks.

Adblock Plus (disabled for ag of course ;p)
DownloadHelper
Australian English Dictionary
Hide Menubar (my favourite)

Edit: wow Net Usage is awesome, just installed it then, just wish it could go in the status bar instead of the toolbars... toolbars are for input, status bar is for output/feedback! :|

Chrome

wish it could go in the status bar instead of the toolbars

I actually like that it can go up the top - no real need for that massive URL text box, so I just put it to the right of that. My status bar looks like a pig with half of the above addons requiring something down there - even if it's not functional (ex: Signature addon, qtl).

Hide Menubar - very nice. Another excellent addition to my already gross collection.

A lot of mine are purely cosmetic, but have grown used to them so like to keep them with any fresh install.

Adblock Plus
Better Gmail 2
Colourful tabs
Ctrl-Tab
Download Status Bar
Downthemall!
Fireftp
Fission (progress update in the address bar)
Flagfox
Forecastfox - Do I need an umbrella today or not (answer - most likely yes)
Foxclocks - this one was particularly useful for my last job, where I had to communicate with different offices around the world. Now I just have one set up to let me know what time it is back in Brisbane.
Java Quick Starter
PDF Download

Netusage and fireftp.

Adblock Plus + Element Hiding Helper ( thanks to whoops list)
Chatzilla
Customize Google
DTA
Flashgot
McAfee SiteAdvisor

cbf listing them all, too many. Click this
http://i296.photobucket.com/albums/mm190/pointyfork/qgl/th_plugins.gif

Scrapbook (save perfect website snapshots, best plugin ever created)
Adblock Plus
Flashblock
FireFTP (full-featured FTP client)
DownThemAll (multi-stream download client)
RefControl (disable referrers)
Download Statusbar
IE Tab (enable IE renderer for Exchange web interface or other IE-preferred sites)
CustomizeGoogle (strip google ads/crap/spying-info)

i dont use any, cant see what for

Adblock Plus
Australian English Dictionary
IEtab
Noscript.