Pandava side
Panchalas (second most powerful kingdom of the Vedic period)
Matsyas
Vrishnis
Kaurava side
Kurus (the most powerful kingdom of the Vedic period)
Madras
Saindhavas (Jayadratha who is brother in law of Duryodhana)
Gandharas (probably a small army as marching a large army into the plains of Kurukshetra from north-west would have been a logistical nightmare)
Bhoja-Andhakas
Trigartas
The vast majority of the armies fighting in Kurukshetra would have been fielded by Kurus and Panchalas (the two strongest kingdoms of the Vedic period).
Distant kingdoms/tribes like Saindhavas and Gandharas would have only brought small armies due to geographical barriers and supply issues.