.sig

A signature file contains a small block of text that can be appended to the end of an e-mail message. A basic signature includes the sender's name and e-mail address. More advanced signatures may also include phone numbers, a company name, title, and other information.

Since signature files contain plain text, they can be edited with a text editor. However, most e-mail programs allow signature files to be created and edited within the program. Therefore, there is usually no reason to open SIG files manually.