Part Time Businesses : Web Design
No discussion of businesses that can be run part time to supplement your main income would not be complete without some mention to web design, which in the last ten or so years has seen so many people around the world boosting their income by dabbling into the murky waters of web design.
If this sort of business attracts you, then first get your own website and a portfolio together as quickly as you can. This might require that you do a couple of cheap ones to start with, for instance approach your local gym or tennis club if they don't have one and volunteer your services, or even for your friends if they work with or represent a business of some sort.
Do try to stick to small businesses if you can rather than individuals as you should find these look better on your portfolio than individual sites, though if they are prepared to pay there is no reason not to do those either. Put some time into marketing your site and submitting to directories as you will get most work this way, as well as through word of mouth - get business cards printed and don't be shy to hand out to family and friends at any opportunity, you never know who they know who will need a website done at some stage in the future!