Are you sure it's a utility?
I think I know what you mean and you can achieve it by taking the following steps (Windows XP):
First you need to create hidden system file:
1. Right-click on the folder and choose 'Properties'
2. Click change icon and choose some icon.
3. Click change icon and choose the old icon again.
Now there's a file called 'desktop.ini' in the folder, but you cannot see it! Optionally, you can create the file by making a new text document and naming it desktop.ini. The file will disappear after you make though.
4. Browse to the folder and add in the address bar: '\desktop.ini' and press the enter key. (For example, if you're in c:\temp, you have to make it c:\temp\desktop.ini)
5. If you did it right, Notepad will open and show the file.
6. Add the following line at the end of the file:
InfoTip=your informational text goes here
7. Save the file and exit.
If you now hover your mouse over the folder, the first line of the tooltip will be what you specified in the ini file. If the tooltip doesn't show, go to Tools -> Folder Options -> View tab and make sure "Show Pop-Up Description For Folder And Desktop Items" is checked.