Search engines such as Google and Yahoo are used by people to find sites relating to particular subjects.
Google is perhaps the best known and widely used search engine. Google AdWords is a very flexible and economical way of advertising. You set how much you are willing to pay per clickthrough and a daily maximum budget. It can be amended, paused or cancelled at any time.
You may also wish to consider the Yahoo! Express programme to get included in the Yahoo directory.
The Open Directory Project database is used by many search engines and portal sites.