If you are into link building on a long term basis, you can exploit following mediums seriously.
- Yahoo 360 Blogs - Since it is a social blogging platform, you get backlinks from your friends. Create a cute and nice girl profile. Make tons of friends and write blogs with links. The entire system gets backlinks on bulk basis and it is internally transferred. More friends you have, more of them give you backlinks.
- Myweb 2, Yahoo - Bookmarking platform, Here again, You can make a lot of friends and generate backlinks.
- Squidoo - In the initial phases your links are nofollow, but I believe once you have enough credibility, you can dofollow links. Here again, the entire website gets a lot of back links and tag cloud helps you to get a share out of that to your pages. Create new pages on regular basis with more or less original content, write rich tag cloud and pray ;-). it's a long term game and if you are into serious SEO business, it's going to be worth it.
- Hubpages - Similar advantage as Squidoo
- FriendFeed - Make tons of friends, share your items and they appear on pages of all your friends.
- Digg - Submit stories, comment on top stories, digg often and early on the upcoming sites who have the potential to reach top page. except 1st, you need to remaining to increase page rank of your profile page and thus give better page rank to submitted urls.