Last May, Yahoo acquired a startup called Inquisitor which offers a search plug-in for all the major browsers. Today, Inquisitor is available as an iPhone app and it shows how certain features, such as Yahoo’s Search Assist, really shine on a mobile device where you want to keep your typing to a minimum.

When you start typing a search in Inquisitor, a list of suggested keywords automatically appears below (just as it does on Yahoo’s regular search engine on the Web). The more letters you type, the more refined the suggestions become, allowing you to select one before completing the word in the search box. Results are presented in large, easy-to-read gray boxes, with favicons and two lines worth of text. Abbreviated news results from two sources appear at the very top if they are available, and can be clicked through to see only news results.

Once you click through to a Webpage or article, it is framed by the app. A toolbar at the top allows you to return to your search, making for more fluid navigation. You can always escape the app to view the app in the iPhone’s regular Safari browser as well. There is also an option for emailing links.

Despite all the advances in mobile Web devices, they are still constrained. Features that simplify navigation or remove unnecessary steps can still be the difference between a usable app and one that you never return to. Inquisitor is one app on the iPhone I will be returning to when I want to do searches, which is often.

Yahoo has already made some minimal changes to the software. The plugin’s integrated Affiliate Links, which have been the source of some controversy, have been removed. And the default search engine has been changed to Yahoo!, though users are still free to choose another engine.

Developer David Watanabe (blog) has created a number of popular applications for the Mac, including the RSS reader NewsFire and Acquisition, a P2P client. He will continue working on Inquisitor, but will not be joining Yahoo! as an employee.

You can find more details on the Yahoo! Blog. And for those who are curious, this application has no relation to TechCrunch alum Duncan Riley’s blog, The Inquisitr.