Some manufacturers ( eg Seiko, Citizen, Casio ) manufactured Wristwatches that could be linked to a computer, ( I remember that in 1982 Seiko showed a prototype of a wirstwatch TV ( monochrome LCD )).
Casio has a broad set of wristwatches, some of them could be descibed as Wearables that can display time among other things, of interest are the
In 1998 S. Mann displayed a Wristwatch videoconferencing computer, it runs GNU/Linux and uses an XF86 Server, there is a pinhole video camera on the watch itself. This device is to say the least impressive, some of the software, is downlable and GPLed.
It is also possible to use a 4 lines LCD display connected to the serial port of your laptop ( numerous paper or online electronics magazine offer schematics ).
PDA's seem to be a cheap and efficient way of sending and displaying data for a wearable, actually any PDA with a VT100 emulation program and a serial link can be used effectively as a terminal ( I have successfully used my HP200LX running kermit
as a terminal for my desktop, giving me an emergency access to it if the display failed )
The distinction between obstrusives an non obstrusives ones is not from the wearer point of view but from the other people, that is is the display forbid to see the wearer's eyes
Some people managed to use blinking LEDs in order to retrieve information from their device. This is one of the simplest display one can imagine : a LED or a row of LED blinking.
Sony sells a device called the Glasstron; in Paris, France the FNAC Montparnasse sells the PVD-V30 glasstron 55 at 13999F ( this is about US $2333 ).
Tekgear manufactures the M1.
With this device a manufactured by Microvison a LASER LED draws the image on the wearer's retina, the US Navy tested it in the summer 1999, at Hawaii ( see in the army now ).
The Microptical corporation manufactures two displays the Integrated Eyeglass display and the ClipOn display.