As explained by Android Police, rooting an Android phone does not necessarily mean installing a custom ROM. Do you want to root the phone but avoid the hassle of custom ROMs?

How to root a Nexus One but keep everything at stock

Rooting Nexus One is easy as it has fastboot enabled. Most rooting guides like this one say the first step is flashing a custom recovery image, but it's actually not necessary thanks to the superboot package by MoDaCo.

1. Download: the superboot package
2. Unzip it.
3. Depending on your OS, edit install-superboot-XXX.XXX and change

   ./fastboot-XXXX flash boot boot.EPE76.superboot.img
   

to

    ./fastboot-XXXX boot boot.EPE76.superboot.img

(basically delete the word flash) This way you won't modify the boot image stored on the phone's ROM.

1. Connect your phone via USB and reboot it into bootloader mode (push the power button whil pressing the trackball). When skateboarding Androids show up and you can read FASTBOOT USB, it's all good.
2. Execute the script you just edited double click the .bat file, or:

   shell ./install-superboot-XXX.sh
   

The phone will reboot using the superboot image. On boot it will install Superuser.apk which is the "Superuser Permissions" app. You can now reboot the phone (adb reboot for convenience) and enjoy your rooted phone. For instance, you can use Barnacle Wifi Tether.

What if fastboot fails?

It's possible that your phone comes with a locked bootloader and so fastboot will fail. Luckily, the bootloader can be unlocked:

fastboot-XXX oem unlock

If this doesn't do anything, check if your USB connection works:

fastboot-XXX devices

If you don't see anything, then you need make sure you can use adb.