I love to decorate my envelopes with fun stamps. Sometimes the post office has colorful stamps like the Simpsons stamps which go on sale May 7, 2009. But I also found a great resource for purchasing older stamps (unused and in ready condition for mailing).
The picture above shows what I have on hand at this moment. They come in a variety of denominations and themes. I order mine from Errol Murphy (firstname.lastname@example.org) Usually I will get $20 at a time. Errol sells them at face value and doesn't charge shipping. The stamps are all usable. You just need to make sure that your stamps add up to the correct postage.
You can email Errol at email@example.com if you would like to order some of these fun stamps. I usually request themes (kids, animals, disney, space, olympics, nature, ocean, Christmas) depending on my mood. I've never met Errol who lives in CA, but having been buying stamps from him regularly for the last 2 years or so. I told him about my blog and asked if I could link to his web site. He no longer maintains a site, but said that people are welcome to email him with their orders or questions.