Overview:
OpenWith adds a context menu handler to the PocketPC file explorer that allows you to choose a different application to open a file than is registered by default.

Instructions:
Tap and hold on any file in File Explorer, and choose "Open [FILENAME] with..." from the menu. Select a previously used application for this extension from the drop down list, or type or browse to a new program to execute.
Arbitrarily complicated command lines are supported. Usually just the program name should be sufficient, however if the application needs command line switches, you can edit them in. The text %1 will get replaced by the selected filename, otherwise the filename will be added at the end of the command.]
NOTE: make sure the program name has quotation marks around it if there are any spaces in the path to the program. It's best to always keep them there.