If an API is not available, you may be able to use an RSS feed. Most websites offer RSS feeds for their content. The biggest drawback to RSS feeds (as opposed to APIs) is that they are usually used to provide a website's latest content in date-or-der. Often times, there is no RSS feed that allows you to pass a parameter, such as a search term or an argument indicating how to sort the results. For example, a hotel site may provide an RSS feed of its best deals this week, but you cannot tell it to give you hotel availability in a specific city on a specific date. However? if a site does have an RSS feed with the content you need, this is a good and easy solution.
Expedia is a good example of a site that provides excellent RSS feeds with parameterization. Check out their RSS feeds here:

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What about a site that doesn't have a good feed? Or doesn't have one at all?
If you find yourself wanting to use content and functionality from a website that doesn't provide an API or a feed, or whose web services don't meet your needs, you still have some choices. First of all, many sites do have APIs that they do not expose except to their partners. There are others that provide them to affiliate networks, so joining such a network will give you access.