"So based on the number of bounce backs in my in-box, plus my copy for each of my email addresses, because I'm in my address book, I think one of my yahoo accounts sent out a spam today. I was able to get into my account, so it's not completely hijacked."
You should change your email password, but that might not entirely fix the problem because most harvesters will use the addresses in your address book and your email address as the sender since it's valid.
For changing passwords, I'll suggest my usual pattern for them. Invent a reasonable 8 character base. It's a good idea to use mixed case, use only letters and numbers for the times when a password isn't allowed to use punctuation and don't make it a word that you'll find in a dictionary (for example, a good base for me might be AbCdVirg). Next, come up with two non-letter, non-number characters like %^ or whatever. Then, to build a password, use your base, then the two characters, then the name of whatever you're using the password for. So, you can come up with a password like AbCdVirg%^etiquettehell for here, which is tough for a computer or person to guess, but all you need to remember is your base and characters, and the rest you'll recall from where you're trying to log in. For sites that don't allow them, you can skip the characters, and if there's a size limit on the password, just use the full pattern and then truncate it to the maximum allowed length. To change it, change your base only and it's still very tough to guess, plus you can write a list of all the bases you've ever used on a card in your wallet for all it matters, since your base alone won't unlock anything.
"I have a method to print them on a spreadsheet with the numbers coded, and I want my kids to be sure to be able to get into my accounts when the time comes."
If you're worried about access to your accounts in the case of your incapacity, then write down your pattern and where your accounts are, and put them in your safe deposit box. If you don't have a safe deposit box, then put it all on an encrypted thumb drive (see http://www.truecrypt.org
for my favorite program, because you can put the program on the drive with the file so you never have to worry about not being able to find the program to decrypt it) and give it to your kids or whoever's likely to end up as their guardian if they're too young themselves. Put the password to that thumb drive in your will, assuming you don't just want to give it to them directly, and even if someone breaks in and steals your will they won't have the drive itself.