Since there are a lot of questions that come back often about mods, i'll be trying to make a unified thread for this, where people go to to get information. i'll start the ball rolling with my own additions, will also ask mods if they can sticky this for more orderly discussion. I won't post any retextures because i am not using any, my pc can barely handle the game at all. feel free to post screenshots of your own favorite retext addons.
Nexus mod managerFirst and foremost, right now if you want to manage your addons, and easily install/uninstall them cleanly, then you need a program called nexus mod manager. It is available at www.skyrimnexus.com and is very easy to run and install. Download the file and then run it to install, then to run NMM, it will be in your start menu and quick launch bar. Then to install mods, browse skyrim nexus, and when you see a mod you like, click on "download with manager". NMM will download it automatically, all you have to do is double click your new addon and it is installed. if you want to uninstall, select your mod and click the disable button if you want it uninstalled temporarily, or uninstall if you want to get rid of it.
SteamworksThis will be released in january toward the end. This will be steam's version of NMM, it will be integrated into the game launcher and allow you another centralized place to download addons. It is not available yet.
skyrim scripts extenderavailable at
http://skse.silverclock.org , many, many mods require this. It is a platform that allows mods to interact with stuff they normaly wouldn't be able to in the standard format, allowing mod makers to make even more awesome stuff. Download the zip file, open it, double click on the skse_1_xx_xx folder, and extract into your skyrim folder. Then in NMM, on the upper right there is a button that launches skyrim with a little downward triangle, click the triangle and select launch skse. Then use that button everytime you start skyrim.
UI modsSkyUI
http://www.skyrimnexus.com/downloads/file.php?id=3863 makes the interface much better. customizable text, multiple, sortable columns, more pleasing to the eye.
Better quality maps with roads.
http://www.skyrimnexus.com/downloads/file.php?id=4929 This improves the map. You can basically rotate the map at will now, on top of seeing roads and terrain up close much easier, plus it allows you to get rid of the cloud cover. There's also a version that looks like a parchment map if that's your thing.
Interface hard coded keys
http://www.skyrimnexus.com/downloads/file.php?id=88 In vanilla, if you try to change some of the keys, they won't work right anymore and tutorials/menus will still show you the original keys. this fixes that.
extra hotkeys
http://www.skyrimnexus.com/downloads/file.php?id=3865 adds 40 hotkeys, making you not forced to use the default vanilla 1-8.
containers categorization
http://www.skyrimnexus.com/downloads/file.php?id=1227 this adds categories to containers you look, mostly useful if you store tons of unorganized stuff in your home chests.
Thuumic :
http://www.skyrimnexus.com/downloads/file.php?id=5626 allows you to use a headset/microphone to use dragon shouts verbally, shouting them yourself.
environment modswars in skyrim
http://www.skyrimnexus.com/downloads/file.php?id=6176 Makes more spawns on the overworld. Now armies of diferent factions will often meet and battle, and often you will be stuck in the middle. brings the world much more alive, and can be used to make the game much harder, especially when you get an automatic 2-3 dragon spawns at the same time
No npc greetings
http://www.skyrimnexus.com/downloads/file.php?id=746 If you are tired of npc's talking to you everytime they walk within 10-20 feet of you, this is for you. you can either disable all the greetings, or tweak the distance so you need to get in their faces to hear whatever they have to say about you looking ill, or being sneaky or strong or whatever.
glowing ore veins
http://www.skyrimnexus.com/downloads/file.php?id=193 Makes ore veins easier to see, especially ebony
Dovahkiin's hideout
http://www.skyrimnexus.com/downloads/file.php?id=6032 It's a basement accessible from every buyable homes, filled with dozens of chests, manequins, display cases, weapon racks, as well as a forge, grinder, and armor table. oh, and alchemist and enchanting tables as well. The whole place is really cool.
subliminal traps:
http://www.skyrimnexus.com/downloads/file.php?id=2653 Makes traps blend in mroe with surounding
essentials be gone
http://www.skyrimnexus.com/downloads/file.php?id=2570 Makes every single npc non essential. If you're evil, now you can kill anyone or anything. Beware though, if you care about quests, be careful with this as you can get stuck forever with quests.
wars in skyrim
http://www.skyrimnexus.com/downloads/file.php?id=6176 Makes more spawns on the overworld. Now armies of diferent factions will often meet and battle, and often you will be stuck in the middle. brings the world much more alive, and can be used to make the game much harder, especially when you get an automatic 2-3 dragon spawns at the same time
This is sparta!!!
http://www.skyrimnexus.com/downloads/file.php?id=1294 Replaces fus ro dah with this is sparta
Combat/game balance modsDuel - combat realism
http://www.skyrimnexus.com/downloads/file.php?id=2700 This mod changes combat o that it's both faster paced and more fun. faster attacks, better blocking, but costing stamina, more powerful power attacks, but costing more stamina as well. Many, many changes.
Sneak Rebalance
http://www.skyrimnexus.com/downloads/file.php?id=819 Makes it harder to sneak. now you'll find that npc's hear you easier, you can't stand in their faces in broad daylight no matter how much stealth you have, and they will search longer and further for you before they give up the search.
Midas magic
http://www.skyrimnexus.com/downloads/file.php?id=3413 This adds new kind of spells to skyrim, beams, insect swarms, cure diseases, and alot more. It also adds new quests
Deadly dragons
http://www.skyrimnexus.com/downloads/file.php?id=3829 Makes dragons much much more powerful, because if a giant or 2, or 4-5 bandits can kick their ass, why would skyrim need a dragonborn to begin with?
Deadly traps and debilitating diseases
http://www.skyrimnexus.com/downloads/file.php?id=2905 Makes traps urt you more and diseases are now something you really have to worry about. you won't be able to carry rockjoint for months withotu even noticing that you're infected anymore.
This list are only some of my favorites that i play with. feel free to sugest more for the folks here who want to be introduced to the wonderful world of mods.