Anything that modifies the data stream is prohibited. Monitoring the stream is acceptable (Lucy Item Collector is a good example of this). Repetative key macros, are frowned upon, but generally acceptable if not away from keyboard. The big no nos, are pretty much anything automated that is not a simple autofire type of mechanism, such as an autofire game pad, or program, and again only while not afk.
As with all things illegal, there are some people that break the rules, and somethings are more aggressively pursued by Sony than others. My advise is avoid anything clearly illegal, and be careful using anything questionable. The worst thing I use is an autofire program to work on skills and such, instead of having to hammer a key non-stop, but do it while at the keyboard, other than the occasional get to go to the bathroom or get a refill or answer the door, or what not, so if a GM does send me a message, I can respond fairly quickly.
The main addons that are completely legal and even encouraged by most, is the use of custom UIs (see the EQInterface link at the top menu bar), and Lucy Item Collector for here, and the other ones for other sites, such as Magelo's. Item collectors simply monitor the data stream and keep records of your personal characters inventory, and sometimes progression, as well automatically update personal profiles and databases, such as the item and spell database here.
There used to be more UI like addons, such as custom help, and journals, and even atlas, but many of those have fallen to the wayside without being updated with the games changes. And one of my favorite addons was a server monitor, that gave live updates to server status, and way back gave current populations (but way back, the eq launcher used to do that as well). The addon was nice after the original patcher quit doing that.
There are alot of programs in the illegal category that do lots of different things. Ones that modify the data stream include, run speed mods, stat modifiers, immediate transportation, etc. Ones that give information you shouldn't be able to know, such as existance and exact location of any and all mobs in a zone, as well as their loot (Although I think they fixed the loot table to not be generated until a mob is dead now, so you can't know ahead of killing it, even with such programs). Ones that automate things more than a simple repetative key stroke, such as using a whole group of players to automatically seek out nameds, kill them, and loot them, without any actual person playing the toons. Alot of illegal programs are simpler, but exploit game mechanics to do things. One of these exploits is called ghosting, where the toon waprs back and forth so fast that it never gets hit, but continues to hit the mob.
I hope this helps some, and you should note, I may be off a little bit on some points, as the standing has changed over the years. The original standing on autofire was no, no matter what the condition, and although I personally haven't seen an official statment about it being ok if not afk, I have heard it second hand a few times, susposedly from GMs, and I've never heard of anyone getting banned for simple repetative keystrokes while not afk. All of this doesn't mean for sure, that it's ok, but you would think it should be, simply to avoid carpel tunnel syndrome.
Yther Ore.