Can I replace the OS of a Samsung Galaxy S? I was attracted by the hardware specifications of the Samsung Galaxy S.

However, I don't like the default TouchWiz skin developed by Samsung. I'd prefer to use the native Android UI, as in the Nexus S and similar devices.

Is it possible for me to perform "self installation" of the stock Android OS, after I bought the Samsung Galaxy S? If yes, how? Will I void the warranty of my phone by doing this?
Yes, flashing a custom ROM will void your warranty.  However, it is very simple to revert to the stock ROM with a Galaxy S for warranty purposes.  Rooting will also void your warranty, but the method below does not require root.

The easiest way to a get a new OS for a Galaxy S is to use Odin for Windows (download here.  Search for "Heimdall" if you want to do this on Linux).  You just need an Odin-flashable ROM.  For the Galaxy S Vibrant, I highly recommend the ROM I currently use, Bionix v1.3.1, which has a TouchWiz-free version.  I assume you have a GT-I9000 however; you should look for one that suits your purposes in XDA's i9000 Android Development forum.

Edit: According to this post, the only completely TouchWiz-free ROM is CyanogenMod, which is unstable/buggy.  So if you don't want CM and just want to replace the launcher, you can do that without flashing another ROM as Amanda says in her answer.  Otherwise you can continue with the instructions below for using Odin.

Once you have the ROM, do the following:


Put your phone into Download mode

Remove the battery
If non-i9000, attach the phone to your PC via USB.
Hold VolumeDown plus either the Home key (i9000) or VolumeUp (non-i9000) and the power button

Reinsert the battery.
Let go of the power button only about 1 second after the white device logo appears (if it does)
After a few seconds a yellow triangle with an android in it should appear, you can let go of the keys since this is download mode.


Start up Odin.
If non-i9000, detatch your phone from USB.
Attach the phone to USB so that Odin recognizes it.
Load the .zip file you downloaded (the custom ROM) into the PDA field of Odin.
Load the 512 .pit file into the PIT field of Odin.
Check "Re-partition".
Hit Start.  Odin will tell you when it's done; if it fails, reboot your phone and try again.  (This happens occasionally).


Note that if you need to back anything up, you should do that beforehand and save the backup to your PC, since everything (except probably the external SD card) will be wiped on the phone.  Also note that you do this at your own risk; Odin is very powerful and misusing it can damage your phone.  I've used it dozens of times myself safely.