I'm working in a software company in India; next month, I'm going to Belgium on a work permit. I want to take my widowed mother with me (I'm the only son).
Is there any other visa apart from tourist visa for her? If so, what's the procedure?
Unfortunately for you and your mother, it seems that she can't qualify for a family visa:
A non EU ascendant can not rejoin for instance his non EU son working in Belgium.
Ascendant means parent, grandparent, and so on.
Note that your situation corresponds with the "second situation" covered by that page, "a non-EU citizen rejoins a non-EU citizen." (In the first case, dependent ascendants are included, but that does not apply to you because you are not an EU citizen.)