There are many "key ring" portable pen out there, but there aren't many engineered as well as an Inka Pen (it's no wonder given that Inka's inventor, Greg Adelman, used to design optical instrumentations for the Scripps Institute of Oceanography and worked on engineering projects for the US military and NATO).
The tiny pen unscrews into a full-length pen and uses a pressurized ink cartridge that can write in virtually any conditions: in wet conditions, below-zero weather or in the hot desert sun. It even writes underwater!
Of all the key ring pens I have used, Inka certainly stands apart. All in all, a very cool pen: Link - Thanks Greg!