At home, I use the Benjamin Moore personal colour viewer. You can download it from the BM web site for $10 US, or buy the CD for $14 US. If you plan to do a lot of painting, it's well worth the money. It's quite easy to use -- basically, you choose a BM colour and select the area that you want to paint the colour that you select. The colours aren't quite true-to-life, but they give you a good idea of what the final product will look like.
At work, I use plain old Paint that comes with Windows. It's not as powerful or as technologically advanced as the colour viewer -- you don't get the shading effect that you get with the colour viewer. But for simple mock-ups, like painting a sofa or a door, it's pretty good. You can use the dropper tool to select a colour from the picture that you're working on, and then use that colour to paint anything in the picture. Depending on what I'm painting, I mostly use the brush tool or the line tool.
Hope that helps. If you need more info, feel free to ask :)