VISIONary
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VISIONary is an Android app that allows you to temporarily root your Android phone, using the 'rageagainstthecage' exploit. It can also attempt to permanently root your device, though results vary.
Discontinued
VISIONary is no longer updated since November 19th, 2010 and the download link no longer works.
Comments and Reviews
I've posted this before, but I might as well do it again for reference:
Apps like Visionary aren't available in the Android Market, so you'll have to install Visionary manually. Once you've downloaded the app file for Visionary onto your computer, you can use the Dropbox Android app to install it. Move the app file into your computer's Dropbox, and then view the file from the Dropbox app on your Android phone. When you tap on the file, you'll be given the option to install it. There are other ways to install app files, but Dropbox is awesome. So yeah.
Also, it seems that the permanent root feature for Visionary doesn't work too well. If Visionary is unable to permanently root your Android phone, check out the section of this article about the G2 and Desire Z: http://goo.gl/c8Bwv