One of my favorite "mean things to do on the web" is to click on those little links some web design firms put on their clients' sites that say "site designed by" or "powered by" (insert company name and link here)...then I visit the designer's site and point and laugh. Better than flaming someone, right?
Don't I sound horrible and cruel? But seriously, I don't know that I've ever seen this "stamp" on a well-designed, professional site. If your site looks like this, the last thing you want to do is draw attention to the fact that it's your work. Some may claim that their clients don't mind this, but I don't buy that. When I started at my first company, the first think they asked me to do to their website was to remove all references to the design company that created it. They did not realize that they had a choice in whether to put it there in the first place.
Putting your name on a client's site is rarely an acceptable practice. The one case where I really think it's ok to add a credit is if you have volunteered design services for say, an open source or community site. Then, you've probably donated the design and perhaps you deserve a link. But still, the more professional way to handle it is to mention your company as a news item or press release on the new site when it launches. For example, "we're pleased to announce the launch of our new site, designed by XYZ Web Design".
Or you could take it a little farther and spin it as a "partnership" project between your company and the client, with some nice quotes about how you worked together to develop an innovative site. If you constrain your "credit" to the launch of the site, it also limits your implied responsibility for the design of the site years down the road when it's horribly outdated and your work has drastically improved!
Just putting your name and link on your client's site (no matter how small or out of the way) because it's part of your contract and that's how you intend to spread your name is simply not professional. Your work should speak for itself. If your client is truly happy, they will spread your name on their own. People will compliment their site and the client will say "thanks, I had this great little company develop it..."
Instead, use your portfolio on your own website, and show off your client sites there with actual links to demonstrate that the site is still being used and it works, and that it's not student work or spec work. A quote from the client can also validate this. And nothing looks worse than when you show a pixel-perfect mock-up in your portfolio, but if you actually visit the site it's a mess. You can't fake good work.
Let your work speak louder than a link that some client was forced to put on their site.