Mam Mam is an understated Thai restaurant on Rue du Marche au Charbon 72, one of the streets running from the Grand Place (south, I think). The owners are Thai and the food authentic, at really good prices (around EUR 10-12 for much larger portions than elsewhere in Brussels). You will find the traditional pad thai and curries (although they are missing a good non-spicy yellow curry), as well as other rice and noodle dish combinations. The service is fast and friendly.
Le 2eme Element is a better bet if you care more about ambiance, as Mam Mam is just a generic, no-frills Thai restaurant in a part of town that lends itself to less people watching than the popular St. Boniface (also with many decent restaurants).