The only mod I've played was faiakes v2.4.
It really is just a large collection of mods. Most of them are very good though. There are a lot of new weapons and items, based on real guns and whatnot, so there's the FN2000, G36, SVD, MP5, pistols like the 1911, Beretta, USP Compact, Walther P99, etc, lots of new armors, ammo types, and a lot of new skins for them.
And the traders sell pretty much everything, so if you have the money you can buy the good stuff and not have to pick it up off a body. The scopes and silencers can be attached to pretty much every rifle and pistol. The silencers are much more useful since the enemy AI is modified so that they don't hear a silenced shot so easily.
No time limit on side quests, the max load you can carry is increased greatly although it is adjustable as an option, non degrading weapons and armor, and tons more.
Overall, I found it to be very good. One thing is that it made the game much easier, since you can buy everything right away, and the weight limit is so high, you can collect tons of loot, get a lot of money and get uber equipment much earlier than you could before. So I'd suggest playing on a harder difficulty than you normally would, or lower the weight limit so you can't get so much money easily.
I prefer it to vanilla Stalker mostly due to the variety of weapons to use, but it makes a lot of good changes.
It is one of the largest Stalker mods, sort of like a total conversion you might call it. Note that it is only compatible up to 1.004 (unless there is a new version of the mod recently).
Another large, total conversion-like mod is ABC Inferno.