Only two are essential, really.
Both by Drog - the Unofficial Patch and the Resolution Mod.
High Quality Townmaps (again Drog :) ) are a good extra to go with those two.
Edit: Changed link to point to the latest version as by 404notfound's post below.

There are only two essentials in my opinion, both of them developed by the same guy. First you'll want this unofficial patch, which fixes literally hundreds of bugs throughout the game: http://rpgcodex.net/phpBB/viewtopic.php?t=39612
Next, you'll probably want this high res patch that makes the game work on modern screen resolutions (download "High Resolution Patch"): http://www.terra-arcanum.com/downloads/section/arcanum
The actual pages for these patches on Terra Arcanum are broken, so grab them from the above links. Note that the version of the unofficial patch on the TA download site is outdated by about a year, so be sure to get it from the RPG Codex link.
Edit: Beaten by a few minutes, but make sure you're getting the latest version of the unofficial patch, released just a few weeks ago.

Thanks ! I've been able to d/l the high resolution patch on Terra-Arcanum, but I hadn't noticed that the unofficial patch was outdated there !

My friend played Arcanum a while ago and encountered a bug that crashed the game whenever his character leveled to level 50 (or 51?). Is there a fix for this?
Also, do you has this kind of bugs have been fixed in GoG releases before?

We should make a Sticky for a mods thread, here.

I remember some big CAR mod too, like a big overhaul.

is the 1.0.7.4 patch included in the gog.com version?

I can't verify this as I haven't bought it yet but it would be very surprising if it wasn't patched to 1.0.7.4.

Considering the game download is a gig, it's possible some of the high-rez stuff is incorporated already. Anyone playing yet?

Unofficial patch and resolution mod are not included.
Downloaded them myself, installed them, everything seems to work just fine in 1600x900 on Windows7.

The unofficial high-res fix is only a 659kb download.

Yes, it is.
I've just installed it and it looks even as it had the hi-res included (although there's no folder for it).

Installed it, with unnoficial patch (091225 version), high res (1680 x1050) and High Quality Townmaps. Works fine (never played the game -just a demo looooong time ago-, though)

Installed bothe the unofficial patch and the high res patch (1920x1200). Everything seems to work flawlessly and I don't understand people who say that it's "meh" graphically. For the moment, I just love it !