You can get the extension at the Classic Retweet website.
To see an example, look at this page where I use the Classic Retweet extension to retweet an item for my class with the #OU3043 hashtag.
So, if you are using Chrome or Firefox as your browser, insteall the Classic Retweet extension and see what happens. After the extension is installed, you will see TWO retweet options; the one on the left is the "classic" option, allowing you to edit, while the one on the right is the Twitter retweet which does not allow editing. You can choose which option to use each time your retweet:
As you can see, the normal Twitter retweet option on the right has the number next to it (indicating the number of times this tweet has been retweeted), while the "classic" option on the left has no number with it.
For me, Classic Retweet is a huge boost in the usefulness of Twitter because I rely heavily on hashtags. When I find something I want to share with my class, I use the Classic Retweet to add the hashtag. I hope it will be a useful extension for you too!
NEXT STEP => Next up is a tool to help you include images quickly and easily in a tweet: Twitshot.