Frazo, "there ain't no such thing as a free lunch." Making money in Second Life is a lot like making money in the real world. Skilled people make more money than unskilled workers. Dedicated workers make more money than lazy ones. Criminals will always find a way to take your money. And ultimately, sex always sells.
With those thoughts in mind, here are (some) ways people make money, and what you generally need in order to start in that business: (Please remember that the current exchange rate is around L$265 to the US$. Also, I will not not be able to give you any HUDs or tell you where to find any of these things. Per current Linden policy, I am trying to avoid naming "names."

No skills required (but not much money):
Camping (find a place that will pay you to sit or walk around on their land. You may have already seen campers sitting on benches, mopping floors or just standing around like zombies. Avoid places that make you pay for the privilege - they're usually not worth the effort). Typical earning: L$1 / 15m. Potential earning: L$20/10min or even higher - but the higher valued ones are usually capped and require you to pay. From personal experience, the web-based one with the big purple tents is *not* a scam. I tested and got paid (it just took a while for the payment to clear).
Web-based surveys (found on freebie islands and many other places, usually an ATM that gives you a link to a webpage). While these ARE a potential source of income, it is my experience that they cost more than you earn in terms of frustration, annoyance and time spent. Typical earning: L$40/10 min survey. Potential earning: L$1000 / sign up (usually a "trial" subscription to some site or service you don't really want, but will charge your credit card or telephone bill anywy).
Travelling: (using a HUD to direct you to places that can make you cash). I know of four versions and have tried all four. One paid just for the act of travelling, however, it is currently undergoing "re-management" and has had issues paying in the past. The other three point you at objects that you can get free L$ from: 'Sploders (objects set on a timer that will pay a portion of their pot to some or all participating residents in the area), Lucky Chairs (if the first letter of your first name matches the letter on the chair, you win the prize) and Money Trees (usually your account will need to be younger than a certain day to get any money from these). Typical earning: L$10/hour. Potential earning: L$1000+ (getting VERY lucky at a "big rollers" 'sploder and getting most or all of the pot).
Some skills or investment required:
(these jobs will require more investigation on your part to find what you can and are willing to do. Because the income spread is so great, I will not be posting typical or potential income. Your Mileage May Vary. Caveat Emptor - Buyer Beware)
Gaming: There are a number of places in Second Life (a HUGE number) that have games requiring varying skill levels to play. Some of these require you to pay to play, and offer a varying set of odds (unseen) to win. Some variants of the game Slingo are especially common, and so standardized that many people rely on them as their sole source of SL income. One particularly prolific producer of these games has a wide variety of games that I have found to be fun and reliable in the gaming experience. Please note that gambling, as defined by Linden Lab, is against the Second Life TOS, so generally there is some element of skill involved in any of the games you might see. Still, just because it exists in Second Life does not mean that it's sanctioned. BUYER BEWARE. I cannot stress this enough. At the very least, stick with the Freeplay model of games until you learn the game and learn how well you score compared to games that have an actual payout.
Model: (Non-sex related) Many places like to have real SL avatars model their clothing, poses or objects for sale. Some modelling jobs are just like camping, but require at least a decent looking avatar or purchasing the items. Where camping is an "at will" job, models often have a set schedule they must meet. Some modelling positions require the model to be "aware" (no AFK modelling) so that they can answer questions or give directions to potential customers.
DJ: (playing music (streaming) in front of a "live" audience). I have not done this, but I understand talented DJs can make decent money, plus tips. This skill requires a good speaking voice, fluency in the language of your audience, and knowledge of the software. A good DJ avatar and your own SL DJ assets are a plus.
Customer Service: Even Second Life businesses need customer service representatives. This may be anything from answering questions to refunding or replacing orders. Customer service is always a live position (differing from modelling), but still usually requires a good looking avatar and regular hours.
Hosting: Similar to DJ positions, hosting requires some knowledge of the topic / region / event, and live performance. Hosts may be responsible for advertising, inviting and greeting guests, handling live games or contests, and basically making sure that the event happens as expected. Pay on these can vary from tips only, to a decent sum, but usually related to your experience.
Dancing / "escorting": Similar to modelling, dancing generally requires a good looking avatar, and may also require that you purchase your own dances and outfits. There may also be certain.... Adult.. elements required. I won't go any further into it than that. Suffice to say, there's always money involved.
A lot of skills or investment required:
(These occupations require a lot of investment - be it time, money or both. Especially where creating content for Second Life is concerned, the more skill, the greater the reward).
Land rental / sales: (HUGE investment required. HUGE loss potential. HUGE earning potential. YOU HAVE BEEN WARNED). Just like the real world, Second Life real estate is a big business. And just like the real world, it has its booms and busts. A Second Life land baron(ess) purchases land, then leases or re-sells it to potential tenants. Since the other side of this (being a renter or buyer) is unbelievably complicated and varying in results, I won't go into the incredible complexity that must be involved to actually have to keep up with weekly rentals or land sales. Do keep in mind that real-world costs for land are on the level of several hundred dollars a month for any serious real-estate person. All of that money has to be recouped through either selling the land for more than they paid, or renting at a high enough rate to cover ongoing costs as well as recover the initial investment.
Building, scripting, textures, sounds and animation: Lots of skill required for all of these - usually tools outside of Second Life that you have to learn, as well as the Second Life system. People who build, script, etc, can usually sell their services or products for whatever the going rate is. Just like the real world: The more desireable an object is, the more people will pay for it.
Good luck to you, sir, and to all other future SL income earners!
Theoretical Quandry - not just another hypothetical question!