Question-Answer sites are the best place to generate backlinks. When you answer a question of your niche, you can place a link to the site. But keep in mind not to spam. These sites act as a great source of information, you can ask your queries here or can also answer questions asked by others.
Popular question-answer site:-
a) quora.com
b) askvilla.amazon.com
c) formspring.me
d) answerbag.com
e) answers.yahoo.com
f) mahalo.com
g) allexperts.com
h) aolanswers.com
i) answers.com
j) friendfeed.com
k) stackoverflow.com
l) shinyanswers.com