Public Service Search is a service that enables non-profit, university, and government web sites to provide search functionality to their visitors without serving ads.
Now this service has been discontinued for new users. It remains functional for those who are already using it. Instead Google is encouraging use of Google Custom Search Engine.
A Custom Search Engine can provide you with free web search and site search with the option to specify and prioritize the sites that are included in your search results. You can also customize your search engine to match the look and feel of your site, and if your site is a non-profit, university, or government site, you can choose not to display ads on your results pages.
If you wish, you can turn off the ads on the result pages. You can do it either when you are creating the custom search engine or through a step by step method described on the official Google blog post.