I'd say ATX was the most popular form factor. Assuming microATX and the like is included in that. BTX also existed at that time however, it never really caught on
ATX
Blast Factor happened in 2006.
The 2006 Hyundai Tucson has a 5 yr./ 60000 mi. basic warranty.
The 2006 Audi TT has a 4 yr./ 50000 mi. basic warranty.
The 2006 Audi A6 has a 4 yr./ 50000 mi. basic warranty.
The 2006 Porsche Boxster has a 4 yr./ 50000 mi. basic warranty.
The 2006 Kia Sportage has a 5 yr./ 60000 mi. basic warranty.
The 2006 Toyota 4Runner has a 3 yr./ 36000 mi. basic warranty.
The 2006 Cadillac SRX has a 4 yr./ 50000 mi. basic warranty.
The 2006 BMW M5 has a 4 yr./ 50000 mi. basic warranty.
The 2006 Toyota RAV4 has a 3 yr./ 36000 mi. basic warranty.
The 2006 Scion xB has a 3 yr./ 36000 mi. basic warranty.