Are you a GSM Galaxy Nexus owner, but for whatever reason you can't pull down the much-anticipated 4.0.4 update? Maybe you're stuck in a low coverage area or you just don't want to spend the data before (or after) you reach your cap. For whatever reason, you no longer need to wait: Google has posted the full ZIP update for earlier versions. Installing is simple, even if you've never done this sort of thing before - but you will need the proper Android ADB tools and drivers installed on your computer. Before you begin, click here to download the English GSM Galaxy Nexus update file, or check this XDA thread for localized versions in other languages or older updates.
Once you've finished the download, go to the Settings menu on your phone. Tap Developer Options, and make sure USB debugging is enabled. Connect your Galaxy Nexus to your computer and unlock it. Copy the ZIP file to the Internal Storage folder in your device. If you've downloaded it on your phone, use a file browser to copy it to the /sdcard folder. Open a Command Prompt or Terminal window in the same folder as your ADB install. Type "adb devices" and make sure that your Galaxy Nexus appears in the list.
Now type "adb shell", press enter, then "su", then press enter again. Type exactly the following, without quotes: "cp /sdcard/7f97fbc19417.signed-yakju-IMM76D-from-ICL53F.7f97fbc1e.zip /cache/7f97fbc19417.signed-yakju-IMM76D-from-ICL53F.7f97fbc1.zip" (If you downloaded another language update, change the name of the ZIP file appropriately.)
Type "exit", then press enter. Type "exit" and press enter again. Now remove your Galaxy Nexus from the USB cable and power it down. Now press the Power, Volume Up and Volume Down buttons together to bring it into Bootloader mode. Press Down Volume twice, then the power button to enter the stock recovery. Use the volume buttons to select "Apply Update", press the power button, and you're golden. Once it finishes, reboot your phone.
XOOM Wifi and Nexus S owners can follow the same basic guides if you find links for the official updates, though the exact method of getting into the bootloader and recovery varies. If you need a non-English update, check the XDA link above; it should have just about everybody covered. Galaxy Nexus LTE owners, you're out of luck for the time being... unless you'd like to try your hand at some custom ROMs.