PROPERTY: Real estate generates good returns principally because the numbers are large to start with so even modest returns on the investment can be very substantial in real terms. It is low risk if you are sensible and crunch some numbers beforehand (to ensure affordability).
If you buy a good house at a good price (mortgage long-term affordable for you even at low occupancy/yield rates) in a good area you will generally be able to rent it easily at a good return. If the property increases in value, that should be treated as a bonus. Get a few houses in same area and options such as hiring agents to find/manage tenants and maintenance companies to look after the properties become cost effective (though even with many houses on rent I still found it was cheap and easy to manage these myself and still work fulltime, the key is to have a list of good trusted tradesmen willing to do jobs for you when you need them and pay them well (above market rate) for doing a good job – it is cheaper in the long run and property is best played as a long-term game).
Buy near places that always have good and constant footfall and preferably long-term renters e.g. near universities (students will rent 1-2years generally) or centers of commerce (office staff typically rent ~6months but will pay more rent and have less wear-and-tear on your house). Favour long-term tenants on cheaper rents less likely to bother you (usually they will appreciate you charge them less and do most minor DIY jobs themselves, plus you will have more candidates to choose from so can be more selective).
WEB: Websites/Online is far more hit-and-miss and frankly unless you are building a mass-market and scalable product/service, ideally something people want, it does not seem worthwhile to me. For better or worse (generally better), you are already competing on a globalised playing field (with more competition, copycats, lower barriers to entry etc.) and without the captivity intrinsic to property. I am sure many aspire to become the next FaceBook or social-network-thingy but that generally seems hard work and risky (er, Pownce.com?). Focus on a disruptive technology product (http://en.wikipedia.org/wiki/Disruptive_technology) making use of new advances as they emerge, ideally combining or converging two or more disciplines, aimed towards a fixed market willing to pay seems a better bet. However, as I gear towards that, I have initially opted for the easy-wins of technically less complex but almost totally automated and revenue generating product-based sites like http://www.sportingglory.com. So far this has been working very well. Once setup, they require less than 4 hours per week combined, and provide long-term passive value without further the need for major enhancement [if they were on a the BCG Matrix, http://www.12manage.com/methods_bcgmatrix.html, they would be cash-calfs ;)]. I am now at the stage I can start having a go at the more adventurous sites/products. Hence, for websites, I would treat them as businesses initially and go for things you know you can build and create revenue from, and convert them into passive streams later in the lifecycle (but you already know that).
INVESTING: Investing (stocks, shares, bonds, savings accounts etc.) has proved to provide constant but modest returns but that is primarily because I am not a risk taker in areas I know little or nothing about and hence have always invested in safe products/funds and stayed away from the stock market. This has by far been the easiest way to make modest amounts of money (4-8% p.a.) for almost zero effort (on my part) as most products can be bought/setup/created in a few hours/days (trackers, bonds, funds) and managed by a fund manager, or minutes (savings), with little more than needing to know what the rough interest rate will be. It is a good way to park cash for a while, especially if you are lazy.
However, given that many of the people with the largest passive incomes on the planet (as well as some of the richest) belong to the world of Finance, I am sure effort spent learning about how to operate and navigate in the Finance world and invest wisely would provide the best forms of passive income I can achieve, but sadly I have no real interest in it.
TRADITIONAL BUSINESSES: Bricks-and-mortar business in conjunction with a partner, and the understanding that later on they will run the daily business while you remain in a supporting role, has provided both the best experiences and passive income I have achieved so far. This works especially well in simple businesses e.g. restaurants/bars, where once things are up and running, they basically run themselves (and they are fun to set up). Eventually even your partner will basically just oversee staff and could run several outlets. If you apply this to several businesses e.g. an estate agents, a law firm, an accounting practice, and a I.T. services company, you unsurprisingly have a series of businesses that literally grow each other, requiring little or no extra time or effort from you.
(OTHERS: I'll save it for another day/thread. Way past my bedtime and it is showing in the writing...)