One option is what is commonly referred to (in the non-web world at least) as a combo box. It looks like a drop down but you can type a value in the box if you want.
Here's a sample page that shows how they work: http://www.thunderguy.com/semicolon/resources/simpleCombo-demo.html
All of the examples except the bottom two let you select from the dropdown or enter text yourself.
Another option is to just use radio buttons with the small set of defaults and then just have an "other" radio button with a text box next to it. This is probably the easier and safer way to go, but does take up significantly more space. It does also allow you to have the short description that you mentioned would be a nice to have.