1. Hubpages.com: Hubpages is a community of articles, or "hubs" on various topics. Their unique system discourages spammers by slapping no-follow tags on outgoing links if your "hubscore" falls below 75. You can increase your hubscore by participating in forums, commenting on hubs, and most importantly, submitting high quality, completely original articles. It takes some work to maintain your hubscore, but the high quality links are worth the effort. I've seen some of my hubs picked up by Google in less than 2 hours.
2. Ebay Blogs: Who doesn't want a link from ebay.com? Ebay blogs are a great way to showcase your ebay products, or just provide useful content to the ebay community. Links posted within your blog are followed, so the potential SEO benefit is huge. My first ebay post was indexed in less than a day.
3. Flickr.com: Flickr is an online photo sharing community similiar to YouTube. Users can upload and describe pictures or artwork. Each Flickr page can be tagged with keywords and can contain a image description with followed links.
   
4. Listible.com: Listible allows you to create resource list pertaining to different topics. For example, I can create a "Top 10" list for Web 2.0 link building methods. Each resource can contain an outgoing link. Each item in the list can be voted on, which determines how high on the list the resource appears.
   

Tips & Tactics
Helpful advice for making the most of this Guide

• When creating web 2.0 links, remember to participate in the community. If you're only there for the link juice, and you're contributing next to nothing, you'll likely be banned or your posts will get buried.