I have recently started to receive WiseStamp http://www.wisestamp.com/ email signatures from colleagues. I think it is essential if you are in business to maintain contact details on your email at all time, both the original email, and the follow up emails.
Wisestamp is a free app you can use in your email signature to promote the various sites you are connected with. A small example of offerings can be seen in the image below.
I tried it out; it took about 30 minutes to set up, and apply it to my GMAIL and Google App accounts. Once it is set-up I only have to change my email in one central location rather than three locations, which is a time saver.
One limitation is that it does not yet send out on my Outlook email, this is a feature that is supposedly coming out soon. I use Outlook at my desk, and access Google Apps when I am working off site. When I access Google Apps email account on the internet it works. You can see from the image below it is quite neat and tidy. However another limitation is if the recipient only receives emails in text, as many people seem to do, the usefulness of this app becomes questionable. I also have a wordpress blog on my own domain name, and cannot yet work out how to add the RSS feed to the email signature. That I think would be quite useful.
You can see a little advert from them at the bottom of the signature, another slight downside of the app, but expected if you are using something free.
Have you tried Wisestamp? Do you like it?