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 Kia Sedona has a 5 yr./ 60000 mi. basic warranty.
The 2006 Kia Sorento has a 5 yr./ 60000 mi. basic warranty.
The 2006 Cadillac Escalade has a 4 yr./ 50000 mi. basic warranty.
The 2006 Chevrolet Malibu has a 3 yr./ 36000 mi. basic warranty.
The 2006 BMW Z4 has a 4 yr./ 50000 mi. basic warranty.
The 2006 Lexus IS-350 has a 4 yr./ 50000 mi. basic warranty.
The 2006 Toyota Camry has a 3 yr./ 36000 mi. basic warranty.
The 2006 Kia Optima has a 5 yr./ 60000 mi. basic warranty.
The 2006 Kia Rio has a 5 yr./ 60000 mi. basic warranty.
The 2006 Mitsubishi Lancer has a 5 yr./ 60000 mi. basic warranty.