Some services offer money in exchange for a pre-set volume of traffic sent to them from visitors clicking a banner on your site. Many individuals lured into these gimmicks by the promise of easy money are soon disappointed. You will need to divert a substantial number of web visitors to their sites to gain any appreciable profit. Of course this is in itself, purely a matter of opinion. It's up to you.
Web rings are another variation on the exchange theme.
Less obtrusive than a banner exchange, web rings are a reasonable alternative. This is a relatively simple illustration. If you are interested in joining a web ring or would just like more information, check out the Ring World site. At over 46,000 rings to choose from, you are bound to find a ring suitable to your site content and theme. If you run a web site that has frequently changing content or features articles of some sort, you can develop an effective rapport with like minded webmasters.