To get multiple signatures in Gmail check out this Gmail Tweak.
There are instructions on the website on how to install it. What I had some problem understanding was that one was supposed to download the script, modify it and use that script with Greasemonkey in FireFox.
It worked good even though the compose page becomes black outside the input boxes. But that is ok until Gmail lets you chose multiple signatures.
Update: Today I found out that the black background on the compose page is a feature that lets you define different colors for different signatures to not mix them.