One under-rated gadget gizmo that really makes me disproportionately happy each time I use it is a return address stamp. So simple and nerdy, and yet so satisfying. First of all, it's faster and more efficient than scribbling my return address out in ballpoint pen - especially when mailing a stack of cards, like invitations or thank you notes - and secondly it's much more legible. But also, stamping it methodically on each bill or invite brings out this latent secretarial fantasy from my childhood (am I the only little kid who loved doing imaginary clerical work??), which makes a simple chore just slightly nicer.
In the past, I've designed my own and ordered a 2" x 1-1/8" self-inking stamp online. But now there are a number of pretty custom stamp options available at online stores like Papersource and Etsy. Here are a few of my favorite designs: