I recently found myself looking into security patterns (the sort that are printed along the interior of an envelope), a nice collection of which can be found
here. I designed my own and thought someone here might enjoy it. Obviously a business/security envelope would be superior, more cost effective, and consumes far less ink. Nevertheless, it's a nice exercise in repetitive patterns, and if you don't have an envelope available or fairly opaque paper to print on, this can help you in a pinch. It is based on a pattern know as Hrungnir's heart.