microSD aka TransFlash FLASH Memory SD Cards

microSD is SD in a SanDisk^{^} "TransFlash" form factor. Commonly used in cell phones, handheld GPS, iPod clones, and expandable USB FLASH memory drives.

About the size of a fingernail: 15 mm x 11 mm x 1 mm or 0.6" x 0.433" x 0.04" - Don't sneeze.

The pins are:

DAT2
DAT3,/CS
CMD,SDI
Vdd
CLK,SCK
GND
DAT0,SDO
DAT1

The DAT1-3 lines start as inputs and become I/O after the SET_BUS_WIDTH command.

microSD supports the MMC serial and SD nibble parallel modes

Can be connected to a standard SD card slot via a mechanical adapter:

See also:

http://www.transcendusa.com/Products/ModDetail.asp?LangNo=0&ModNo=96 MicroSD cards with data sheets available.
http://en.wikipedia.org/wiki/MicroSD
http://www.molex.com/cgi-bin/bv/molex/jsp/products/datasheet.jsp?channel=Products&print=y&productid=99268 Molex 1.10mm (.043") Pitch TransFlash Memory Card Connector, Push-Push Type
http://www.mouser.com/search/ProductDetail.aspx?R=500873-0801virtualkey53810000virtualkey538-500873-0801 TransFlash Memory Card Connector RA, SMT, Push-Push type by Molex^{^} are in stock at Mouser for ~$2.50 each
http://jae-connector.com/en/general_en.cfm?series_code=ST2&contact_number=&s_height=&l_con=&r_con= JAE Low profile/ SDATM "ST2 Series" for microSDTM Memory Card
http://www.mouser.com/search/ProductDetail.aspx?R=SV1R007V1Avirtualkey65610000virtualkey656-SV1R007V1A MICROSD CARD SOCKET by JAE Electronics^{^} are non-stock at Mouser for ~ $5 each.
http://www.opencircuits.com/MicroSD/Transflash_Socket A component file for the Molex push-push socket in Eagle PCB CAD format
http://www.howardforums.com/archive/topic/1023327-1.html A bug in some third party 1GB microSD cards can prevent formating with a 16KB cluster size. Use a 32KB cluster via XP command prompt: FORMAT E:/FS:FAT /A:32k where E: is the driver letter.