I have disabled my pay per use data plan. I only use wifi. I use DropBox. I can take camera-phone photos and store them in DropBox or I can upload photos from my PC to DropBox. When I have wifi, I will sync them.
You can use DropBox to upload/download/sync any type of file, including music, videos, documents or apps. The most recent version even has a lockcode to give you some additional security in case you lose your phone.