Lets discuss some Game MODs & also Game Modding.
There are some cool Mods for all GTA Series of games. Here's some MODs for which are easy to install:
GTA San Andreas
1. Map (Island) Mod:::
=>"Welcome To Akina", Japan Map MOD .:: like 2f2f
its a small island near "Santa Maria Beach" in Los Santos, its got more than
15 HairPin turns.

a delight for the
Drifters 
.

I DRIFT in my Dodge Challenger R/T

its really FUN.
=>Theres also a "ALCATRAZ ISLAND" map mod. (where the film "The Rock" was shooted)
Installation-
Just Put the provided two files in "Data" folder. thats it. the Island is installed.
2. Car Mods::
I've installed lots of cars, after driving the Cheverolet Camaro SS in Most Wanted, i just fell for these Muscle cars too.
i've Installed a Camaro SS, Dodge Challenger R/T, BMW 7 series car, BMW M3 GTR (yes the Most Wanted one),........

Dodge Challenger R/T

BMW 760i
Installation
Its a bit tricky, You've to do some NotePad editing & replacing TWO TEXTURE files in "gta3.img" file- the heart of GTA game.
But some car mods have a script in them, so using "GTA SA Mod Installer (SAMI)" the process is automated.
TOOLS Required
IMG-Tool ,
GTA SA Mod Installer (SAMI)
For the Game MODs go here-> BEST one=>
GTA-Filefront ,
GTA-WorldMods,
THE GTA PLACE
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Need For Speed Most Wanted
Installing Custom Vinyls
=> Download
MW TEX.
=>Open MWTex & right click inside it, & select "load". ->now select the "Vinyls.bin" file of the particular car.
example=>in Most Wanted Game Directory, goto "
CARS->BMWM3GTRE46->VINYLS.bin".
=> now select the required Vinyls, you want to replace it with.
Example=> if you want to put this vinyl- "
B&B Tuning M3", it comes with two files ==> "BMWM3GTRE46_UNIQUE01.dds" & "BMWM3GTRE46_UNIQUE01_MA" (MA=MASK).
Now in MWTex, after selecting VINYLS.BIN form BMWM3GRE46 folder, search for these two files & then Replace it with the new ones.

Thats all. Your Vinyl is Installed.
Dont forget to take a BACKUP of the FILES before modding