I'll consider Blog commenting first, then consider forums (OK, pedants may argue that should be fora - I argue language changes! Now where was I ...) and the Web 2.0 sites such as HubPages and Squidoo.
Blog comment spamming (adding useless comments for the sole purpose of getting a link back to a website) is still very prevalent. There are blackhat automation tools that can do this at the press of a button. You can recognise the comments very easily: "Nice post", "Great thoughts" or similar positive (to encourage the comment to be accepted) fluff followed by a link.
As a result many blogs are set to "nofollow" for comments. This means that the 'weight' of the link is zero to the search engines. The spiders may follow the link but it will have no impact on the PageRank of the linked page.
So should you bother trying to Add Comment Links on Blogs?
Absolutely - if you do it in the right way. The usual principles apply - quality is more important than quantity and the if you contribute in a useful way, you will get more traffic. Aim for the higher PR sites and dofollow if possible.
How to Add Comment Links
The easiest way is to use software tools. There are two that I recommend:
It is unbelievable that (at the time of writing), Comment Kahuna is free. This is a very powerful program developed by one of the most successful internet marketers, Jason Potash. Comment kahuna will take a search term and find places where you can post comments. It searches Ezinearticles, blogs, lenses and engadget.com, providing you with PageRank information.
It will then load the pages in its own browser and allow you to post. But even your posting can be automated. You can set up profiles so that your name, email and web address, together with a skeleton comment, can be pasted into the comment box by clicking a button. The links above will take you to a page with an overview video.
[If you like Comment Kahuna, you should have a look at some of the other Kahuna Products, all of which get excellent reviews. The signups for some of these products are often restricted and there is great demand when the doors are opened.]
Market Samurai does many things, including helping you to find site where you can leave comments. This link will take you to the section on Market Samurai Promotion.
Commenting on Forums
BoardTracker Discussion Search
BoardTracker is a very useful Discussion Search Tool. Here is what they say on their website:
"BoardTracker is an innovative forum search engine, message tracking and instant alerts system designed to provide relevant information quickly and efficiently while ensuring you never miss an important forum thread no matter where or when it is posted.
Boardtracker brings the most targeted audience closer to the boards, by being a search engine only for boards and by supplying a categorized and highly effective searching and browsing experience to users."
BoardTracker allows you to enter your keywords into your profile and will then email you. There is also a firefox plugin.
Web 2.0 Commenting
Look at my Web 2.0 Marketing page for tips on gaining backlinks from Web 2.0 sites. The fundamental principle is: you need to participate and be a member, not just a creator of sites to get backlinks.
Same principles apply at the article sites too.