If you're a regular reader of link building  content then you're likely no stranger to great link building companies like Buzzstream  and Ontolo , and you probably know about a lot of the best link building blogs , but the reality is there is a lot of great link building content "hidden" within more general SEO and online marketing blogs.
Here are five great examples:
1. Ross Hudgens Link Building Posts 
Ross has a lot of interesting link building tips (like building links with Google alerts ) and some of the best (best here meaning "hardest hitting" and most effective at eliciting actionable advice) interviews on the subject of link building. He also writes in-depth on important and somewhat underdiscussed link building topics like evaluating directories  and how to value a link  in a way that you can gain specific, usable insights (or even develop a process).
2. Viper Chill's SEO Posts 
Another blog that doesn't focus solely on link building but offers some excellent and unique tips is Viper Chill. Glen Allsopp  offers a lot of interesting advice around link building within his blog's "SEO" category, including creative tips for using article submissions , and he puts out really in-depth guides on topics like guest blogging  and (you guessed it) building backlinks . Again there is a lot of very detailed, actionable, and unique advice in these posts.
3. SEO Moz's Link Building Posts 
It's certainly no secret that SEO Moz has a ton of great SEO content up on their blog, but if you're looking to jump right to the link building goodness you can check out their link building  posts. One of the great things about the content here is that you get a lot of different takes and approaches as there are contributions from the Moz staff, their SEO agency partners Distilled, and YouMoz.
Similarly to SEO Moz, Search Engine Land and Search Engine Journal offer a variety of different voices on the topic of link building, including thought leaders such as Julie Joyce of Link Building Company Link Fish Media , Debra Mastaler, Eric Ward, Jennifer Van Iderstyne, Garrett French, and others. Having a mix of different perspectives from a variety of very smart link builders means you can get a lot of value and knolwedge out of one feed.
Also worth mentioning here for those of you looking for more link building resources: the folks at Vertical Measures (who author a great link building blog in their own right) put together an awesome list of link building resources  worth checking out.