While mapping has been extensively covered but always from either the Virtual Tabletop perspective, not the on-the-fly-mapping idea. Basically what software could you use to map a grid-based dungeon in real time? It doesn't have to look pretty, the current tool in use is Excel with square cells & cell boarders. However the 'right tool for the right job' person in me pulls my hair out every time I hear this, so I want something better. It must be at least as fast as excel, support marking of secret & normal doors and such. I plan my dungeons on grid-paper, so rectangle only is fine for now.

This is just for old-school mapping: Not a battle mat, not for online play. Just something that is easier to share then scanning in a sheet of grid paper.

No Mac or iOS solutions. Android, Windows and Linux are what we use More obscure OS programs are also allowed, due to the fact we are huge nerds.