If you're looking for a guide on how to root on Windows, there is a fantastic guide here written by W0lv3r1n3!!
I know there is a stickied guide for this but it hasn't been updated since 2.3.1 so I decided to update it a little bit just to make things easier for people. Credit for this goes to Amin Sabet and DistortedLoop with a few changes/additions by me.
Warning: If you break your phone trying this, I am not responsible. I do not guarantee that the following is safe or that the steps are correct. Rooting your phone voids the warranty. I have no responsibility for anything that occurs as a result of your following these directions! I cannot provide any support nor guide you in undoing this process and restoring the phone to factory condition.
This process will wipe your phone. Ie, you will lose all your apps/data on your phone and have to reinstall them. It will also wipe your external storage, so back that up prior to proceeding if you want to keep anything.
1) Download this file (http://dl.dropbox.com/u/22368847/adb-fastboot.zip) and unzip it to your desktop (Inside you'll find all the files you need to proceed including "adb", "fastboot", "recovery-clockwork-18.104.22.168-crespo.img", and "su-22.214.171.124-ef-signed.zip").
2) On your phone, go to Menu>Settings>Applications. Select "Development" and check the box next to "USB debugging"
3) Connect the phone to your Mac via the USB cable
4) Open a Terminal window on your Mac (Applications>Utilities>Terminal), and copy/paste the following into the Terminal:
~/Desktop/adb-fastboot/adb reboot bootloader
5) If your Bootloader screen says "LOCK STATE - UNLOCKED" then skip to Step 6. If it says "LOCK STATE - LOCKED" then copy/paste the following into the Terminal:
~/Desktop/adb-fastboot/fastboot oem unlock
6) Copy/paste the following into the Terminal window:
~/Desktop/adb-fastboot/fastboot flash recovery ~/Desktop/adb-fastboot/recovery-clockwork-126.96.36.199-crespo.img
For the following steps while in Recovery, use "Volume Down" to highlight a selection and "Power" to choose that selection:
8) Choose "mounts and storage"
9) Choose "mount USB storage". Your phone should now be mounted on the Mac as a Device called "NO NAME"
10) Move the zip file "su-188.8.131.52-ef-signed.zip" from the "adb-fastboot" folder to the "NO NAME" Device on you computer.
11) Eject/unmount your phone in the Finder on your Mac.
12) On your phone, choose "Unmount"
13) Choose "mount /system"
14) Choose "+++++Go Back+++++"
15) Choose "install zip from sdcard"
16) Choose "choose zip from sdcard"
17) Choose "su-184.108.40.206-ef-signed.zip"
Some people with new Nexus S' are finding that the file discussed in the following steps does not exist. If you cannot find it but can still reboot into Clock Work Recovery Mod then don't worry, you're still rooted and fine!
At this point you have root, but your phone will continue to replace ClockworkMod Recovery on reboot. To prevent this, do the following:
1) Reboot your phone, and download the app "Root Explorer" from the Market
2) Open Root Explorer and navigate to the "etc" directory.
3) Tap on the gray box that says "Mount R/W"
4) Tap and hold on the file "install-recovery.sh" and rename this file to "install-recovery.sh.old"
Updated for ICS and included instructions in the zip!
5) Download the app "ROM Manager"
6) Open ROM Manager and choose "Flash ClockworkMod Recovery". At this point you are all set .