This week I helped a client start the transition to Gmail from Microsoft Outlook. Turns out that although he used the same email address, he did have different signatures setup for various uses (personal, business, charity, etc). With Gmail, it is possible to setup a separate signature for different email addresses (which not many people do with Gmail), but not choose from different signatures.
As I thought about his problem, I figured out that he could easily meet his need to have a signature chooser by using a feature from Gmail Labs called Canned Responses. By enabling Canned Responses then creating as many signatures as you want to, adding a customized signature is very viable and easy to do.
Watch the video below to see the instructions!