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I prefer to use backlinkwatch.com as it is free to use and it provides accurate information.. other sites like a href would charge you for using..

In Penguin updates, those sites were penalized which were involved in building unnatural links or low quality links, so to recover you need to check your overall link profile, if you have use the same anchor text again and again, you need to correct it so that it would look natural.

As we know, nofollow is not followed by search engines while dofollow is followed by search engines. These days, all are equal, Google knows each and every thing.. if it is for link building, make sure you are doing naturally.

I would say, searching in Google with the keyword "top 10 press release sites" would solve your questions..

Well, guest posting is a method of link building and traffic.. guest posting or guest blogging is known as the most effective method of link building today.. you must use it for higher ranking in Google search engine.

Page title is very important part of SEO, you should use main keyword in the page title, make sure your page title reads like a sentance for better result...

If you are getting good result with your current site without a sitemap then it is good, no need to use sitemap if your site is very small.. Sitemap is mainly needed by big sites where thousands of posts and pages are there.

Blog commenting can better than social bookmarking I would say. Using blog commenting you would be able to build natural links to your website as there are many platforms of blogging software are available, you can put naked link anywhere you want.. with social bookmarking you need to use keywords which is risky now.