They're both quite small, and they were both made over a year ago, but apparently that's just the time scale I've settled into since working on TNM - everything is glacial for me now.
Hassat Hunter's "Expanded Phasmatis Apartment" mod simply overwrites the original version of that map to make the formerly fake doors in the apartment building accessible to inquisitive players. In other words, it's extra content to explore for a little bonus B&E action. Who knows what you'll find when you break out the lockpicks and start sticking your nose in other people's business?
Chris the Cynic's "Link Mod" addresses the common complaint that the three Forum City hub levels are too large compared to how much content they contain, requiring tedious treks across town to access common locations for quests or to restock your equipment. The solution is to add "links" (little blue orbs) from the subway station of each district to the various points of interest throughout that area. At each destination, you'll find a corresponding link that takes you back to the subway if you click it. Verily, Hypertext shall never cease to impress.
Each addon page contains instructions for installing or uninstalling the mods and further explanation of what has been changed and why. I hope these additions will let you squeeze a little extra enjoyment out of TNM before you gorge yourself completely on the fruits of this year's Steam sales.