* Solar Panels. In the UK, the government pays you per kWh generated. Needs an upfront investment, but stable income and you reduce your own energy bills. See http://shkspr.mobi/blog/tag/solar
* Renting out property. Needs a much larger initial investment, and requires ongoing maintenance. On a good month I only hear from the managing agent once per month with details of how much rent is being paid.
* Stoozing. Find a credit card with 0% interest on balance transfers. Stick the money into a savings account. Or, if you like risk, on a horse. Need a good credit rating and decent savings rate to make more than a few hundred a year though.
* Affiliate marketing. If you can find a good theme (e.g. Halloween) and a decent place to put the links (somewhere that wants them - not spamming them everywhere) it's possible to get a moderate passive income. Well... not quite passive, requires upfront time commitment of selecting links and locations.
* Hosting / simple websites. Again, little bit of up-front commitment. Works best if you can find a local niche of people who want a friendly face to host their websites. Usually as simple as buying a domain, setting up WordPress, and turning on automatic updates.
In general, the best passive schemes require a large initial outlay and/or a dubious moral outlook. A better way to enhance your income is to eat out less, stop buying stuff you don't need, and shop around for deals. Boring but true!