I've been on Twitter now for about three years, and even now I come across little tidbits, tips and tricks about the microblogging site that surprise me or astonish me when I learn they've been around forever.
One such item is the 'modified tweet,' or MT. Many of us, myself included, have glossed over an MT notice in a tweet and just assumed it was a mistype. Actually, this is a very useful tool when you want to retweet someone but have to make a major edit to an original tweet and wish to retain credit.
Here are the basics: Nowadays if you want to retweet someone you have a very helpful button underneath the tweet in question, and in other people's timelines you will see a green tab in the corner of the tweet with your screen name beside it, then the original tweet verbatim below it.