From today until Friday, Microsoft will be giving away $25 to use toward apps in the Marketplace. In order to enter the contest, you must follow @windowsphone on Twitter and then tweet them the name of your favorite app using the following phrase:
"@windowsphone My must-have #WindowsPhone app is _______."
Each day of the contest, Microsoft will choose eight winners to receive $25 credit toward apps. Be sure to include the hashtag #WindowsPhone and reply to @windowsphone. You can check out the full contest rules on the source link below.
Source: Microsoft (Thanks for the tip, Aaron!)