Your circumstances
Firstly, regardless of your circumstances, make sure you are getting the benefits that you are eligible for. Secondly, surveys, clickthroughs and cashback are ways to make assured but limited gains from the convience of your home PC. Thirdly, Beat the Bookie, Beat the House, Financial Spreadbetting Offers and Let-A-Room are tax-free, risk-negligible and available to all (if you are renting the latter is subject to your landlord approval) and offer a decent £11k tax-free between them. Fourthly, if you have a reasonable pot of money, get into shares/funds and property - higher risk for higher returns, and the largest tax-free earnings. Finally, if you want a shot at unlimited gains, consider your own business.
In debt / no savings: the three proven routes to wealth (shares and funds, property and your own business) all require a level of capital, so disregard them for now and focus on paying off your debts and amassing some funding. Beat The Bookie, Beat The House, and the Let-A-Room scheme together offer a potential £10k tax-free, and are available to all with negligible risk. Also, consider if you can make a one-off windfall from selling personal belongings (up to £6k tax-free), downgrading your car or selling a car if you have more than one (tax-free also).
Got capital but risk-averse: Shares and funds, property, and your own business, all involve some risk, but the potential gains from these income classes far exceed the other income classes above. If you are risk-averse, consider investing in index trackers (e.g. mirroring the FTSE 100 performance) rather than individual shares, consider investing in a property fund (a range of property holdings), rather than an individual property, and consider setting up a low-outlay business along side your paid employment. Don't shy away from the big three because of risk-aversion, instead do all three and spread the risk that way.
Got capital and happy with some risk: the big three again, starting with shares/funds and property. Shares and real estate both produce excellent long-term returns, far in excess of cash. Crashes happen in both classes but history has shown that keeping nerve and holding throughout still ensured profits that exceeded cash investments a few years later. A long-term positions in both will produce strong returns. Whatsmore, both property and stocks have the largest tax-free incentives. Put in the £7200 maxi shares/funds ISA each year and all gains are tax-free. On top of this, you have a £9600 personal capital gains tax free allowance. On property, all capital gains on your principal property are tax-free. Buy strategically: homes that you can add value to or homes in new hotspots. Sell, profit and move up. Or alternatively build up a buy-to-let portfolio, researching carefully the yield. That brings us to self-employment. A low-outlay one-man internet business probably offers limited earnings at less risk. If you're aiming bigger than this, work within your means - don't risk everything on one venture.
Time-rich wannabe: keen to ditch your job, retire early, or just downshift as soon as possible? Then working flat out on making a success of your own business isn't for you. Occasional trading within a portfolio of shares, funds and/or property is more like it. First, start a multi-pronged attack on paying off your mortgage, using a mix of the earnings sources on this page, together with smart cost-cutting from our Save Money pages. Use your work breaks and commuter time to make money: share trades, cashback, surveys, clickthroughs. With no mortgage and lean expenses, you'll be able to ditch your employment and switch to a mix of tax-free earnings comprising the likes of matched betting, casino bonus bagging, let-a-room, and shares and funds. All from the convience of your home or PC, which can be wherever you want.
Cash-rich wannabe: Self-employment is the key to unlimited gains, but until your business is making those sorts of gains, the best approach is a muliti-pronged attack. Keep your job, set up a business on the side. Get into shares/funds and property as soon as you can. Use your work breaks and commuter time to make money: share trades, cashback, surveys, clickthroughs. Be tax efficient: use all your allowances, such as matched betting, bonus bagging and let-a-room.
Students: rather than getting yourself into debt or taking a nasty low-paid job, consider funding your degree using beat the bookie and beat the house - both are tax-free. As a student you are still eligible for tax, so you could also do Cashback, Surveys and Click-Throughs or your own internet business (such as Zlio or Ebay) up to the tax-free £5435 threshold. That's £10k+ tax-free per annum from the convience of your PC.
Retirees on State Pension or low pension: you can make money whilst receiving your pension but will be taxed on your income, with your state pension counting towards that income. That means your income tax allowance is gone, so instead consider Beat the Bookie, Beat the House, Financial Spreadbetting offers and the Let-A-Room scheme, as they are all tax-free. If you own a property consider downsizing as the capital gains are tax free.
Homemakers and wannabe-full-time-parents: if it's a choice between being a full-time parent and struggling with only one income coming into the household, or working part-time or full-time but spending most of the earnings on childcare, then making money from your home PC offers you a way out. When the baby is asleep or the kids have gone to bed, pick and mix from beat the bookie, beat the house, clickthroughs, surveys, cashback and more. This way you can be a full-time parent and homemaker without the financial constraints.