Windows Phone homebrew app like DoubleAt, by Jaxbot
Download releases @ http://windowsphonehacker.com
Licensed under GPLv3. In (unofficial) laymen's terms, you can use this source for any purpose, so long as your software remains under the same license. You can use the use privately for any purpose, but if publicly released in binary form, your modification of the source must be made publicly available under GPL: http://www.gnu.org/licenses/gpl-faq.html#GPLRequireSourcePostedPublic
This project also contains Heathcliff74's WP7 Root Tools SDK, licensed under proprietary terms listed here: http://wp7roottools.com/