To run Thief the Dark Project and Gold perfectly on modern systems? Have an issue that needs fixing? Want HD resolutions and 4K capabilities? Want modern keybinds? This is the one patch to end all patches that is constantly being updated. You will never need any other.

I take no credit for this mod. All credit goes to Voodoo47 and the people working on NewDark for making this incredible patch that has helped us all for years.

You can find the download link here (note all latest versions appear here as well, ModDB versions are out of date): http://www.ttlg.com/forums/showthread.php?t=134733

Here you will find all available graphics mods for the game. All should be compatible with TFix, unless extremely old.
If I'm missing any feel free to mention them in the comments and I will add them here.

Gecko/Bentraxx's 'HD Mod' for Thief Gold and Thief 2: http://www.moddb.com/mods/thief-gold-hd-texture-mod/downloads

Necrobob's 'Necroage': http://www.ttlg.com/forums/showthread.php?t=144312

TFix already includes the Enhancement pack, but in-case you want to install with NewDark here's Nameless Voice's EP2: http://www.ttlg.com/forums/showthread.php?t=141621&p=2189796&viewfull=1#post2189796
(Thanks, H3H3!)

Other helpful or interesting mods can be listed here. Feel free to mention some to me and I'll add them here. Excludes fan missions.

Child of Karras' Sound Enhancement Packs: http://www.ttlg.com/forums/showthread.php?t=119405
QuadX's Ultimate Difficulty Mod: http://www.moddb.com/games/thief-the-dark-project/downloads/ultimate-difficulty-mod-for-thief-1gold

To sum it up: Thief FMs are fan-made missions for the game. These can range from short 5-minute missions to extensive campaigns that could last hours! TFix comes with a fan mission loader by default. To have it start up at every game launch, press properties on Thief Gold, launch options, then type in '-fm' (without the apostrophes). You'll need to have a place to put your fan missions so in the game's folder, create another folder for only your fan missions and name it whatever you want. Open up TFix's fan mission loader and direct it to get fan missions from there. Place all of your fan missions in the folder you created. Do NOT unzip the fan mission files. Leave them as .ZIP files when placing in the fan mission folder.

Now that you've set up your loader, you might be wondering where the missions are. Well, there's no need to worry! There are plenty of good mission resources out there. From my personal experience, these websites are completely safe. If you want some fan mission recommendations, you can always find some on the Steam discussions as there are many posts about them. Enjoy!

Thiefmissions: http://www.thiefmissions.com
Taffer's Paradise: http://www.taffersparadise.co.uk/thief1missions.html

If you have any questions or are confused about something or your game has issues, whatever, feel free to ask and me and I'm sure others from the Thief community will do our best to help.

If you're new or confused about something, you might be able to find help here. Here's some tricks and tips to help you git gud. Some might be common sense, but I've likely put them their since I've seen someone confused about it before. You can find a full guides worth of this here: http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1130213333

1. Crouching and walking off a ledge will result in a silent landing, no matter the surface.
2. Noise made is dependent on your speed of movement, not by whether you're crouched or walking or not (e.g games like Deus Ex). If you slide down a hill and start moving very fast, it won't matter whether you're crouched or standing, you'll start making more noise.
3. Tapping the forward button while crouched and in 'walk' mode (as opposed to run) will allow you to traverse any surface silently. Don't hold for too long though or you'll make noise. Very helpful on louder surfaces.
4. Gas arrows and more so gas mines have large splash damage big enough to get more than one person if hit near their feet.
5. Dropping items from a small height will be very quiet, as opposed to dropping them from a large height or throwing them.
6. Even if a guard is fully alerted, if you're in pitch black and manage to whack him in the back he will be knocked out.
7. Servants can be knocked out whether they're alarmed or not.
8. Activating holy water and shooting undead with your water arrows will blow them up in two direct hits.
9. If you see a large group flies and walk into them, they'll zap you.
10. Food will give you more health than usual if eaten after taking a healing potion.
11. The weird berry foods in later levels will restore lots of health.
12. The cameras in Cragscleft (second level) can be destroyed by shooting them in the lens with a broadhead arrow.
13. Wooden doors can be destroyed by continually whacking them with your sword. It is extremely loud though.
14. Blocking makes noise if you're near a solid object as it seems to hit it, so be careful.