Umm ... the schematic editor of my CAD. One could probably even design a wedding cake with that. It lets me draw on the symbol layer (or any other layer) in all sorts of shapes. For some things such as flow chart arrows I have made symbols, for others I just pick from template sheets.
Often I have to mark up a client's drawing or schematic where I either do not have the source or can't read it in. Then I use MS-Paint to copy and paste stuff in, make connections, and such. As simple a program as MS-Paint is, it's been rather useful.