Second Life has a lot of business opportunities. Residents can earn money in many ways, including employment as various professions/jobs (dancer, DJ, etc), opening a store/vendor (separate or in a market with others), buying/selling land, scripting, building, etc.
Second Life Business
Because of the ability to exchange Linden Dollars for real world currencies, and vice versa, some 60,000+ users of SL earn "positive Linden flow", i.e. the money they make in their activities in SL exceeds their cost of playing so that they earn some amount of money converted to USD to their paypal or bank accounts. There are thousands of individuals who earn a significant percentage of, or all of their real life income via SL based businesses, some earning in excess of $2000 to over $5,000 per month. There is a wide range of businesses operating in SL, such as:
There is a wide range of media opportunities in SL, from virtual newspapers and magazines, either as inworld objects or web-blogs focused on SL news, to machinima in the form of movies or tv-format entertainment, as well as radio stations and DJ operations.
Marketing and Advertising
The difference between marketing' and advertising is that goal of advertising is to sell more of your products, while marketing goal is to inform people that your product suits their needs. Marketing includes advertising as a part of marketing practice.
Business owners can promote their services and goods in numerous ways, both on web and in Second Life. There are a bunch of companies who provide professional promotion services (check Advertising Networks).
Some steps be performed manually (also read the Advertising for Free article).
Second Life advertising opportunities include traditional services like web banners, e-zine publications and website promotion, as well as SL-specific advertising: adboards, SL Search promotion, social person-to-person advertising, profile picks and many more.
Listing your business, goods and services
It's a good idea to list your business in major business listings - search Google for second life business listing to get the largest ones and submit your business there.
If you create and sell SL stuff (any stuff, buildings, scripts, textures), list them at SL marketplaces. The most famous one is [XStreetSL], former SL Exchange. They have been bought by Linden Labs in 2009.
Unwarranted Business Opportunities
These business activities are currently illegal and forbidden to perform and advertise in Second Life: