Q: What are we talking about here?
A: Some players have taken matters in their own hands to address the countless bugs left in Fallout 2 after the final official patch was released, and have compiled and released packs of replacement scripts, fixed maps, updated data files and the like. Being unofficial add-ons they count as mods, which is why this resides in the Modding forum.
Q: Which are these patches, then?
A: At this time we're primarily talking about the 1.02+ patch by killap, which contains almost exclusively bugfixes and touch-ups. There are also the 1.03 and 1.04 patches by Seraph (from which killap's patch derives some elements) and the 1.03, 1.04 and 1.05(.1) patches by Celestial; however, in addition to fixes all these contain various arbitrary changes with a tangible impact on game balance. The 1.05 patch is also known to contain a potentially game-stopping bug of its own. Most people will agree that killap's patch is the best choice, hands down.
Q: How do I get hold of these things?
A: They're available in the downloads section of No Mutants Allowed. You should also be able to find links to killap's latest updates in his signature.
Q: So what is this thread for?
A: Since killap started working on his patch, it's been used for ongoing bugspotting and progress reports. If you need help with a technical or gameplay problem that happens when you have an unofficial patch installed, post about it here (with all the relevant details as usual).
Q: What about the other patches? Are they still being worked on?
A: No work is being done on the Celestial line of patches (including the announced and de-announced 1.06 patch). Seraph updated his patch to 1.04 in Nov 2005, but as far as bugfixing goes it is essentially made redundant by killap's patch. You can post questions, problems and insights related to the older patches here, but seeing as their creators are not active here and the contents are partially problematic, it seems likely that interest in them will wane. Resident knowledge about the specific details of these patches will for the most part be limited, so if you have a problem, don't discount the possibility that no one will have a ready answer for you.
Q: Why can't I post in the regular gameplay forum if I'm having one of these patches installed? After all my problem could very well have nothing to do with the changes.
A: It might not, but more importantly it might. Given the complex nature of an RPG like Fallout 2 even a small change could have subtle and unpredictable effects, potentially invalidating any knowledge or advice relating to the unmodded game. You owe it to the people who offer to help to provide them with any information needed to determine if they can provide an answer and should take the time to do so; playing a modified game definitely qualifies as such a circumstance.
Q: Where can I find more information on my own?
A: Regarding Celestial's patches you can look in this thread. It is rather long and cluttered, but it provides the background and near the end of it you can find some discussions on bugs and problems. Killap's patch has been discussed extensively in this thread which eventually grew a bit too long and has been replaced by this one.
Q: Is there any reason why I shouldn't immediately grab and install some of this?
A: Well, if you're new to the game I actually recommend that you play the game at least once without any mods or unofficial extras. This way you know you're getting the "real" Fallout 2 experience, and, if nothing else, doing so lets you post in the gameplay/tech forum should you need it.