Geek Shop

Linux and Tux shirts.
http://geek-shop.net/

FreeToBeGeek

Geek focused Linux shirts.
http://www.freetobegeek.com/

OffLine

Featuring MMORPG, computer lifestyle, and fantasy shirts.
http://www.offlinetshirts.com/

Load Rage

Retro gaming designs.
http://www.loadrage.com/

Nosediver Tech Apparel

Designs for programmers, linux hackers, and open source coders.
http://nosediver.com/

NTKmart

Anarchic geek designs for those that Need To Know. UK-based.
http://www.ntkmart.co.uk/

Hackerstickers Clothing

T-Shirts and hats for geek guys and girls.
http://www.hackerstickers.com/products/geek-clothing.shtml

Fraglesstees

A small selection of computer, geek, and gaming shirts.
http://www.fraglesstees.com

SysWear

Humorous computer-oriented t-shirts.
http://www.syswear.com

Openstuff

Linux/Tux, Debian, and FSF shirts and ties, plus other open source clothing and novelty items for European geeks.
http://openstuff.net