Vanity postage. This pretty-easy-to-use website will generate official US postage stamps featuring a photo or illustration of your choosing (works well with iPhoto for instance). When the service was first introduced two years ago it was abused by clever pranksters -- my favorite hack was a Unabomber postage stamp. The Post Office shut the program down. The service is available again but I don't know what the actual image limits are (vs. their stated policy). We made some silly faces and they made stamps from them. I've seen them used for wedding/party invitations. You can also put business logos on them, or other non-political messages. The cost is more than twice that of regular stamps (less if you buy them in bulk). A 20-stamp sheet of these makes a nice personal, utilitarian gift.

-- KK

Photostamps
20 stamps, 39 cents
$18
Available from Stamps.com

Two other sites offer a similar service for the same price. They offer mild design differences in the bar coded postage area.
PictureItPostage
Zazzle (integrates nicely with Flickr)

For Canadian Postage:
Canada Post Picture Postage

All three of the following English-speaking countries offer a photo stamp on a sheet next to a postage stamp:
For U.K. Postage:
Royal Mail Smilers
For Australian Postage:
Australia Post Personalised Stamps
For New Zealand Postage:
New Zealand Personalised Stamps