Nowadays, attachments are looked at with suspicion, and rightly so. Scan everything you send with a reliable virus scanner, and do the same with attachments you receive. If you don't know an attachment's origin, don't open it. Chain letters suck also. I won't even go into the superstition aspect of chain letters, except to say that if you believe these annoying little e-mails have any magical powers, then get offline and send the money you used to send to your ISP to your psychic friends instead.
Chain letters are bothersome and as much of a waste of bandwidth as commercial spam.
Signature files can be useful and sometimes cute or amusing, but don't quote a paragraph of Aristotle or a screenful of Steven Wright one-liners. If you send me more than one message, then I've already seen the thing and don't want to see it again. Use some judgment. I know, some of you look down your noses at chatters, but hey, it's a viable, and vast, area of Internet usage. This means you, AOLers.