The 26 bit H10301 format is by far the most common card format out there.  It's versatile and works with nearly all access control systems out there.  It's specs are as follows.
Frequency: 125 KHz
Number of Bits: 26
Facility Code: 1-255
Protocol: Wiegand
Card type: Proximity
AuthorizID carries many cards, fobs adhesive tags and wristbands in the 26 bit H10301 format.
For more information, check out our easy to understand tech blog on the H10301 format.
For more information about the chip in H10301 cards try this article.