The coolest project I've worked on was for a certain country's Navy. The project itself was cool and I'll talk about it below but first, even cooler than the project was the place were I worked on it.

I would go to this island off the coast where the navy had its armoury. Then to get into the armoury I'd go through this huge tunnel excavated in solid rock.

Finally, once inside I would have to go thru the thickest metal doors you've ever seen to get to crypto room, which was a tiny room with a bunch of really old men - cryptographers - scribbling math formulae all day long.

I can't give a lot of technical details on the project for security reasons but basically it was a bootable CD with a custom Linux distro on it. Upon booting up the system would connect to the Internet looking for other nodes (other systems booted with that CD). The systems would find each other and essentially create an ad-hoc "dark net".

The scenario was that some foreign force would have occupied the country and either destroyed or taken control of the Navy systems. In this case, some key people would boot these CDs in some PC somewhere not under foreign control (and off the navy grounds.) This would supposedly allow them to establish secure communications between surviving officers. There is a lot more to it but that's a good harmless outline.

As a bonus, I got to tour an active aircraft carrier :)