I am developing several websites and I usually place a 'created by' link at the bottom next to copyright to promote my job.
Is this a valuable link? Even if it comes from completely different sites (e-shops, forums, blogs, company pages etc)?
Actually, Google is looking at site-wide or template backlinks negatively these days. I do not happen to agree with this fully. It is traditional to give credit to those who have helped you with a link in the footer. But because of the excessive footer links used in template designers in the past few years, Google, and rightly so, has determined that this was an SEO tactic designed to game the system and it is. Therefore any link in a footer if it appears once is fine, but site-wide has caused severe penalties.
Case in point. I got a call a few years back from a template designer who was being penalized for links in the footers of the templates he designed. He was trying to get as many of these links removed as possible. He mentioned that if the link was broken and the citation remained that all should be well. And it was in time. I kept in touch and the penalty was lifted but the damage was done to his business as he nearly dropped off the face of the earth, as far as Google was concerned, for many months.
Evidence: https://support.google.com/webmasters/answer/66356?hl=en Quote: Widely distributed links in the footers or templates of various sites
Why footer link diversification is horrible advice.
It seems to be all the rage these days to justify site-wide links in footers again as long as there is diversification. However, Google stills says no. (See the first link above.) Here is a warning shot over the bow of anyone who want's to try this tactic.
J.C. Penny was slapped with a huge Google penalty simply because of links created in the footer of sites. It was an ad bug of course and it cost J.C. Penny dearly. J.C. Penny and other companies where completely de-listed through the entire holiday season making the final salvo Google's. It sent shock-waves through the SEO community that they better clean-up their act and fast. Google was not here to play.
Site-wide external links are dangerous. More to the point Google is looking for these style of links in headers, sidebars, and footers. Don't do it!
Footer links carry limited value in regards to link juice but may bring in relevant referral traffic.
My suggestion is that you use you
a) Make this link appear only on the home page and not a site-wide link
b) use your Brand Name as the anchor text in order not to over optimize your backlink anchor text profile which may result in a Penguin penalty.
Minimal SEO value as it is a sitewide link but if you've built a great website which credits you in its footer then it's surely going to be positive from a branding / potential new lead generator perspective.