I have been dabbling in affiliate marketing, and as GlasgowGirl says it can be a useful small income. I place banners and links on my website that earns me money on a pay-per-click, per lead, or per sale basis.
Recently I have also been creating webpages on the Squidoo webiste. Squidoo is a simple way to create single webpages. It is made up of a large community and is completely free. You do need basic HTML knowledge to add links and banners to your pages, though the Squidoo community is always happy to help beginners. Once you have created your webpage, you can visit the Squidoo forum to request member reviews and receive that all important traffic.
Squidoo is an excellent site for those new to webpage building, and an excellent addition for the more advanced.
The only drawback to Squidoo is that the majority of the members are based in the US and my webpages are aimed at those in the UK.
I would love to hear if anyone knows of any similar sites to Squidoo, especially any geared more towards people in the UK.