Particularly on a mailing list. Indulge me in this analogy:
Picture someone coming into your home one day, armed with a megaphone. They walk up to you and start to read out their signature, speaking through the megaphone. Annoying huh? Now picture them leaving and heading to the house of one of your colleagues and repeating the process.
Hang on, isn't that the same thing as "direct mail" (aka: spam)?
Now, you might forgive someone for doing this once (maybe they are a newbie), but when they next have something to say, and do it again, it's really really annoying.
Nettiquette suggests that you should not use more than 4 lines for your signature. I agree, but (after a few years experience) I would even go further; you don't need a signature at all, except perhaps during an initial round of communication with a new client. Why? Once they know who you are, shouting your name and other junk at them isn't interesting.
If you really need to advertise which company you're working for on an open source mailing list, set the Organization: header in your MUA so that it includes that information in the meta data. There is no need for any other information. If someone wants to know more, they will email you and ask.
If you have a .sig, please please please get rid of it, at least when you post to mailing lists.