Monali,
As a general rule, I strongly discourage your submission to either link directories, article directories or blog directories. I would also discourage your linking to your site from various other websites you create be they from tumblr, blogger.com, wordpress.com, etc. These methods as you are describing them at worst would be deemed manipulative links, at best would add little-to-no value to your site.
Instead, create the maximum amount of value you can for users on your site. Create content that others will WANT to share, tweet, like, +1, and link to. You did not share your site URL so I'll use the Joomla extension concept you mentioned. Let's talk about that for a moment. Take your extensive knowledge of Joomla and think about how site owners wish to extend the core functionality in ways the software does not support. This gap creates the market for extensions.
When we look for extensions to add to client websites, we have a few considerations:
1. Who is behind the extension? Some extensions are produced by a single individual, others by development companies, and still others are produced by larger companies. For example, Janrain is a major provider of social integration software and they produced an extension for WordPress. The support they provide for their extension is world class. They have a ticketing system where experienced techs promptly respond. On the other hand, some extensions are produced by individuals or companies who are completely non-responsive to support requests.
2. How much experience does the producer have with the extension's purpose and the platform. Ideally, you want to work with an author who is experienced in both Joomla and the particular area (let's say an SEO extension produced by a SEO company for example). Is this extension the first one produced by the individual or company?
3. Joomla has extension standards. Were these standards met and is the extension listed on Joomla Extension Directory (JED)?
4. Does the extension fill a gap which adds value? Let's say you make a 404 log extension. That adds value by offering site owners a way to understand more about broken links. The basic extension can be greatly improved by adding more features such as:
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having the 404 log accessible via the admin panel rather than a regular log file which is otherwise difficult to read
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having the 404 log update live
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capturing the referring URL so the site owner can see where the link came from
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capturing the search term used (practically an outdated concept now, but still...)
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possibly automatically redirecting users to the most closely related URL
Such an extension could add real value to users. If you review posts on Joomla.org or various SEO and forum sites, you may step into a conversation where users are discussing problems and add value to the conversation by mentioning the extension as a solution. The site can also categorize extensions by their ratings and other factors.
Understand Joomla already provides an extension directory so what added value does your site offer?
My recommendation is spend 100% of your "link building" time and effort addressing the above areas. If you do a fantastic job, you will EARN better links then you could have ever built yourself, and none of the earned links will be viewed as manipulative.
TLDR - build VALUE, not links