Penultima is a famous series of Neverwinter Nights modules designed by Stefan Gagne.
This campaign consists of, at the moment, 5 modules. There's a new series in works, titled Penultima ReRolled, which likewise consists of 5 modules (at the time of writing the 4th part has been released). There's also Penultima Unlimited, an online campaign - the game sessions are scheduled at neverwinterconnections.com, I know nothing else about them.
The modules have been included in the Neverwinter Vault's Hall of Fame. However, since the series is fairly old, people nowadays sometimes ask how the heck they got there, they aren't that amazing. New modules tend to set standards... However, the modules are still fun and are recommended for everyone who plays NWN. Mandatory reading, so to speak. Also, this, along with Stefan Gagne's other works (Elegia Eternum), show that he's capable of making extremely interesting modules.
The modules are quite light-hearted and humorous in content, yet they are not there just to give grins. They have lots of action, some puzzles, and unforgettable characters.
They are somewhat simple in structure and each part can easily be played in one sitting. (I've played three days and I'm now in latter half of the last module...) Despite of their simplicity they're still, for some odd reason, very enjoyable.
The game is set in a distant corner of Forgotten Realms that nobody, obviously, has never heard of, mostly because, completely unlike in every other place in Faerûn, nothing worth mentioning ever happens there. There's some mines. There's some farms. One slightly boring magic university. That's it. (I wish the next edition of FRCS puts this place on the map...)
The series starts from Penultima City, which is "part 0" of the series. The area has training facilities, some stores, a cute dragon, and the Guild of Middlemen that serves as the portal to the adventure. You can, if you want to, also play each module separately, but this module is very convinient nevertheless.
Pest Control is the modest start of the series. A dwarf called Glod Gloddson, who is developing an open mining system with Tucks, has a little goblin infestation problem in the mines. So, there you go. Kill goblins. He pays handsomely for that. Of course, while you're there doing the most boring quest you've done in your life, you discover something far more intriguing: A lost temple and a child with magic staff...
Dewey's Decimal is an adventure set in the local magic university. Your job is to navigate the magic library (every Pratchett fan knows libraries are dangerous places) and find a thesis paper from there. There are a few interesting subquests, including one cool music quest... This adventure is probably the second to best of the series. (Libraries are cool.)
In Hazard Pay you meet a very annoying elf guy who wants you, a complete nobody and expendable adventurer in his opinion, to go in a forest with weird magic in it and recover the remains of a lost god. The end is good in my opinion, but the rest of the module is boring - but the point of the module was get the player to be annoyed and play the next chapter to get a revenge or something. Yep.
The module Clucking Hostile will make you forget the tedious forest romp and try a less tedious countryside romp instead. Now, your job is to find a magic scroll from a cave under a stretch of farmland. Sounds easy? Not really. The animals in the farms have taken over. The corporate people of the farm are turning into nasty zombies. This module was fun - cool names, ridiculous enemies (chickens casting Burning Hands while I did the same on them... Woohoo), fun action, and numerous Apocalypse Now references. This is probably the best part of the series in my opinion, along with the second part.
The last module, Home Sweet Home, is the epic finale that has some quite annoying puzzles and unfair encounters, but otherwise, it has been so far interesting - less fun than other parts though. I can't say for sure until I finish the module (in a few days). This module also has shades of Elegia Eternum in it, by the way...
Overall, it's not the greatest mod series ever, but definitely worth playing. I've so far gained 2 levels and got lots of silly but cool items to hang on the walls of my little home cave in Orsraun Moutains. And the series even has a cool easter egg in the first part. Whee.
You can, at the moment, get these from Neverwinter Vault (http://nwvault.ign.com/), or the web page of the campaign (http://pixelscapes.com/penultima/).