Running a custom ROM? Or just rooted stock? I'm assuming you're on 2.2.
If it's a custom ROM you might be able to enable the "move all apps to SD" feature before following the next paragraph. If it's rooted stock then look in the market for an app called apps2sd. Follow the instructions in the app if there are any.
Or, and this really bites, go to Settings> Applications> Manage> All tab> check every single app you can to see what can be moved to the SD card. In fact, you're probably better off hitting the menu softkey and telling the phone to sort the apps by size and move the largest that you can to the SD card.