Can you sign someone else's name legally?

Yes, if you have a 'Power of Attorney.' This is often set up if a person is not available for a specific event, or item. I've drawn these up for people that need to sell a house and the husband is deployed to Another Country.

Without a Power of Attorney, it would not be legal on a document. People do sign for their spouses all the time though, that's not legal either, but it only matters if their spouse has a problem with it.

Actually, it is legal for spouses to sign for each other. In the eyes of the law, a married couple is a legal unit. This is why one spouse is liable for debts of the other, even if the other spouse incurred the debts on his/her own and without the partner's knowledge.