How to add a simple and reliable search field for Drupal?

Some content management systems (CMS) do not provide good searching facilities. However, a good search is very helpful. There is a simple and free solution: Using the Google site: search parameter. Thus, the high quality Google search engine can be used.

The results are shown on Google. The Google results contain only pages form this domain.

Alternatively, there is the Google Custom Search Engine (CSE). This technique allows to show the results on the same homepage without changing to the Google site. However, CSE is more complicated and restricted.