How can I tell what workspace I am in with Unity?

13.04

This functionality is now inbuilt within 13.04 and later - the launcher workspace icon shows which workspace you are currently on:

12.10 and previous

10.04/10.10/11.04/11.10

To do this, first install the PPA:

sudo add-apt-repository ppa:geod/ppa-geod

Update and install:

sudo apt-get update
sudo apt-get install indicator-workspaces

12.04 onwards

The PPA author does not have a preprepared package for 12.04 and later. However, you can download the same deb-package used in 11.10 and install that.

Download the v0.6.2.2 deb package

Install the prerequisite:

sudo apt-get install python-wnck

how to start

Finally add indicator-workspaces to your startup-applications (search in Dash for this) if the installation hasn't already added this:

Ubunutu 14.10 (Based off of fossfreedom's answer, sorry I need more rep to comment)

1. sudo apt-get install python-wnck
2. sudo apt-get install python-appindicator
3. wget http://ppa.launchpad.net/geod/ppa-geod/ubuntu/pool/main/i/indicator-workspaces/indicator-workspaces_0.6.2.2_all.deb
4. sudo dpkg -i ./indicator-workspaces_0.6.2.2_all.deb
5. indicator-workspaces & disown
6. (Add to startup as fossfreedom mentions above)

If you do download the latest version from http://ppa.launchpad.net/geod/ppa-geod/ubuntu/pool/main/i/indicator-workspaces/ make sure you grab the required wnck libs/bindings in synaptic or the software center. Otherwise this indicator will not work.