The most popular online mods..  you've already mentioned Action HL,
which is an excellent mod.

Day Of Defeat - http://www.dayofdefeat.com
Counter-strike - http://www.counter-strike.net
Firearms - http://www.firearmsmod.com
Frontline Force - http://www.frontlineforce.com
Natural Selection - http://www.natural-selection.org

TeamFortress Classic, Deathmatch Classic - Both worth checking out, and
come already with the HL 1.1.1.0 patch

- I think those are basically all the most popular mods.. there are of
course hundreds of others... although you may have a harder time finding
servers for those

(By the way .. you MUST have the 1.1.1.0 patch before you can play any
of these mods online! .. if you don't have this patch, use google to
find it on the internet..  do not use the sierra auto-updater)

The mods all have their own install utils, and once they're setup,
you're basically ready to go (you'll probably need to configure controls
and all that kinda stuff) .. and yes they all use basically the same
interface

--
Ben Cottrell AKA Bench

"Computer games don't affect kids; I mean if Pac-Man affected us as
kids, we'd all be running around in darkened rooms, munching magic pills
and listening to repetitive electronic music." - Kristian Wilson,
Nintendo, Inc, 1989

Tour of Duty

http://www.tourofdutymod.com/

Frank proclaimed...

Hello Frank. Note that many of the online mods are persistent and have
no real conclusion as such. You'll be playing against other people who
will invariably be better than you which makes it seem tough and
(sometimes) annoying.

With these mods quite often you'll need to practice. If this is the sort
of challenge your looking for then all well and good. If not stick to
the single player mods.

Having said all that, don't let me put you off. Many of them are great
fun.

Pirates, Vikings, and Knights is pretty fun. Not as serious as CS or
DoD.

http://www.pvkmod.com/

Many thanks to you all for, once again, posting such useful
responses.......I'm off to download some!
Cheers
Frank