Guest blogging is a great way to get good links for free. You just have to find people willing to let you guest blog and you have to write descent content.
Blog commenting still works but it's not nearly as effective as it used to be. Web 2.0's are great ways to build links, but again they aren't as effective as they used to be.
Social bookmarks are great on the good sites such as SU, Digg, and Reddit if you can get some votes. The best way I've found to SEO for cheap is to create a bunch of web 2.0 blogs and build them up and link to your website.
Also don't forget social media. Hit up Twitter, Facebook, and Youtube to get traffic quickly as SEO takes time.