I just started messing around with it and over the years (and many Google Help searches later) I have figured a few things out.
You can do just about anything with Photoshop. I have made business cards and printed them out at wallet-sized photos, thank you cards:
(Print as a 4x6 and write with a Sharpie on the back. We did these with a Wedding Photo as a Thank you then too.)
(Print at 8x10 and frame. Letters can be found here.)
Digital Scrapbook pages (Costco will print these at 12x12 for $2.99 in Colorado):
and latest, Birthday Invitations (of which I will make matching thank you cards after the party)
I love that you can personalize everything and print out a 4x6 for $0.13. It doesn't get much better than that.