Blog Comments and Social Bookmarks

Blog Comments
These are links created with anchor text being taken from the “name” you enter when you place a comment on someone else’s blog.
Typically low value links as far as Google is concerned as they place less value on links at the bottom of pages like this.
Having said that, it’s also possible to find (and buy lists of) high PR pages where comments are automatically approved which means you can get relatively high PR links easily.
Blogs like this can be found using software such as Scrapebox which performs searches on blogs know to allow the auto approval of comments. These lists can then be checked — again using Scrapebox — to determine their PR and select target sites on which to leave comments.
The problem with auto approve sites is that they can quickly become overrun with spammy links from other people which can lead Google to devalue the links on that page but alternatively, if you’re happy to make comments that actually add value to the conversation, then you can target blog posts where the comments are approved (and therefore no spammy comments will get through).
Also with auto approve blogs you also get a lot of other outbound links on each page. This means that the link juice of the page will be divided more thinly between each of the linked-to websites. The fewer existing comments on the page you are dropping the link on, the better, as this will maximise the link juice being passed to your website.
If you are using auto approve blogs, then you can also benefit from well-optimised anchor text by using your target keywords in the “name” field.
If you are targeting blog posts where approval is required, it’s recommended to use an actual name so as to not appear spammy and increase your changes of having the comment approved and published.

Social Bookmarks
Social bookmarks are sites such as Digg.com where users submit their recommended links for others to also view. These are good links to get and I have a hunch that, because they are social-based, you can get away with dropping larger numbers of these links than you might be able to with other types without setting off any link-velocity red flags.
You can either submit these manually on the better quality sites such as Digg, Stumpleupon etc or you can also use service providers and software to drop larger numbers.
Although the pages from which your site is linked to on the social bookmarking sites are relatively low authority, they do benefit from having a low number of outbound links (OBLs) and therefore the link juice from the page isn’t divided between other websites.
The nature of social bookmarking sites means that there is a relatively high turnover of content and therefore the link juice you receive will diminish relatively quickly compared to other forms of link building. The structure of these sites and the fact that link juice diminishes over time also allows the more recent social bookmarks to enjoy more link juice initially though, which is what makes them a good link type.
Although it’s not something that I use, there is software available for the automation of social bookmark submissions. When I do social bookmark submissions, I prefer to use service providers who run the above software themselves.