"Using the APIs from your Flash or Server Side framework couldn't be simpler. If you know how to make an http request, and how to process a JSON response, you are in business," says Mark Lucovsky. Here's a simple example for web search:
There are some differences between the old SOAP API and the REST one.
- the new API doesn't require a key
- there's no limitation for the number of queries
- it's much easier to use
- you can use the REST API for web search, but also for image search, news search, video search, local search, blog search and book search.
- you need to send "a valid and accurate http referer header"
- you can only get up to 8 results in a single call and you can't go beyond the first 32 results
It's interesting to notice that Yahoo's search APIs are more developer-friendly and, although they require an application ID and have some usage limitations (5,000 queries per IP per day), they offer more features and they are more flexible, by also including XML output. Another important difference is that Yahoo doesn't require "a valid and accurate http referer header".
Philipp Lenssen suggests that it's much easier to just screenscrape the results, but search engines could change their code or block your requests.
Update. Check this excellent interview with Mark Lucovsky, who mentions that the API has been available for almost two years, but it wasn't officially documented: