This Instructable is specific to making an electronic viewfinder for the Canon S95, but assuming your digital camera has video-out capability, and you can find the correct connector and pin-out, you can adapt for your specific needs. This viewfinder is USB has rechargeable batteries and can be recharged via a microUSB connection.
This is the Mark 1 version, with a Black & White LCD video module scavenged from a Wild Planet Video Spy Car. This viewfinder is larger than I wanted it to be, and is not color. The next version will use parts scavenged from a Vuzix/Icuiti Video Eyewear/Glasses unit.
The other components are avialble online and make this project fairly easy with only minor soldering required.
Why would you want a viewfinder for a digital camera when you have that nice LCD display on the back? It might be easier to shoot in the sun for one reason, but who really cares why... if you want one, here is a way to make one.
Step 1: Get Some Parts
I bought the following from sparkfun.com:
1 - Lithium Polymer Charger
2 - 3.7V 110mAH Lithium Polymer Batteries
I scavenged a small switch from a previous project to have a power off/on for the viewfinder.
I had a small plastic box from some old Polaroid Kids Instamatic camera film that fit almost perfectly.
I also ordered a mini HDMI adapter to hack and use to physically stabilize the attachement to the camera. The Canon S95 has a mini HDMI port just above the Extended USB port. I used this mini HDMI port to give the viewfinder a more solid physical connection to the camera. There are no electrical connections to the mini HDMI connector you see in this Instructable.