While I may love many, many things about Android, that doesn't mean that it lacks any annoyances. Of those annoyances, the inability to mount the SD card in the phone and PC simultaneously ranks close to the top of the list. Thankfully, there are apps like Dual Mount SD Widget to save the day.
What Dual Mount SD Widget does is quite simple, but extremely helpful: it allows you to mount your SD card to your phone and PC at the same time. This means that you'll be able to transfer files to and from your phone via USB without losing the ability to run apps that are stored on your SD card. Of course, in order for this widget to do its thing, you need not use the built-in mounting option, and you must disable Android's auto-mount (but the widget includes its own auto-mounting option).
The app will set you back $0.99, and you can grab it by hitting the widget below.