Barter is the name of this process, and it can be an economical alternative if you're strapped for cash and need some work done. Bartering won't work for everyone. Obviously if you need someone to be in your office eight hours a day to answer the telephone, it's going to be tough for someone to trade with you for that type of work. On the other hand, if you need some help marketing your new product or service, for example, you may be able to barter with a marketing consultant who would be willing help you write an advertisement or a press release in exchange for something from you. Be creative in your exchanges and don't discount personal expertise that you may have outside your business that could be valuable in an exchange.
Internships can be a great way to get workers at little or no cost.
However, in this case, you are expected to offer something appealing for the potential intern, like learning a new skill or getting exposure to a certain industry. Students are going to use your job as a stepping stone and they expect a lot, since you're getting their services for free, in most cases. If you want someone to do your filing or type letters, an internship probably isn't the answer. But if you're looking for an assistant to help you do research for clients or help organize events, you may be able to interest a student in coming to work for you. The best way to get started is to call colleges and universities near you and ask how to set up an internship and what the requirements are.