Dolibarr is used by 1 million of users (so not as much as Odoo with 16m users).
Similar to Odoo in a lot of point but full opensource, more affordable than Odoo and easier to use and customize. Has less external addons (1200 vs 40000) than Odoo but external addons are rarely required. Odoo would be a better choice if you have a very large company with an internal team of developper, Dolibarr will be easier and cheaper if you have less than 5000 employees.
Disclaimer: i'm on old Odoo integrator, now Dolibarr developer.