Submit to thousands of directories and get massive backlinks.
Blogs are a great place to build links from.
Social Bookmarking
Hundreds of links guarantied from over 40 different sites.

You can build great backlinks from forums without being spammy.
Link Exchange
There is an almost unlimited number of people willing to trade links with you.
By giving out free articles about your field containing links to your site you gain loads of links.

Automatic Generated
There are services that let you put links on your site automatic and in return get loads of links automatic.
Paid Links
The most effective way to get quality links is undoubtebly to pay for them.
Why choose one link when you can have all of them?