At a small agency I used to work at, we included a footer link for any projects we completed for a service trade or for non-profits as a way to drive leads.

I noticed in Zoe Rooney's contract sample, she has a clause for credits where removal of the footer credit is an additional cost:

We may, at our discretion, include a small footer credit on the sites that we code containing a link to our site… You may not remove that credit line without written permission from us, as long as you are using our code on your site. If you would prefer not to include that credit, please notify us in writing (email is fine) so that we can discuss the cost for removal.

I usually put a footer link to my website. I don't think is too much invasive!

I think it depends on your customer. Its generally more professional to not list it and humans.txt is a great method.

That being said : when I ran my own agency, we had a "designed by" on many of our sites and it drove notable amount of business and in some cases, clients requested it. Apparently, some businesses want to advertise that they hired a firm or professional to create their website.

For instance, we were based in Atlanta, GA and were somewhat well-known in the area, many of our customers were proud to have hired a local firm so they wanted it visible.

Another example : Michael Heald, the great designer behind Fully Illustrated.com usually has a credit mark on sites he designs. He is wonderful, if I hired him I would want to tell visitors too.

I think it depends on the project, but in most cases putting a credit in the footer seems kind of inappropriate, and I prefer to put it either in the code, or as a humans.txt.

I think in the case of a creative project or collaboration of some sort, attributing yourself and whoever else is involved is cool though.