The first time I heard about Tupper's self-referential formula my mind was blown. The idea of an equation that plotted itself on a graph was wild. When I took the time to understand the formula, I realised that it was just plotting a 106x17 pixel grid that was essentially a means of decoding a bitmap. The formula lost its charm a little when thinking that the section above it was identical other than the binary equivalent being incremented by one (i.e. having a little extra dot at the end).
None the less, I think about Tupper's formula every now and then for the potential puzzles it could be used in. Imagine receiving a postcard addressed to Jeff Tupper and the number below written on it.
7456558717625143833614876830355342230424609112706188186156194763379977292648380019523613686783084587304733127301392969186960873114415737315052284287957163154075204825902258908233816790773018999920813900719633223605331722335212629047269409087912159094740121808363467348997712072794155774924443943388314442529540012651725442751853438324069061565159860822415392061366891606496503782624469301517643577920239831052217971499286769660980064737598820492055031373434551475904143975594175720979665309425437115713599832064