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
NOTE I wouldn't use this plugin in practice, because it defaults to a dropdown with javascript disabled instead of a text box (bad accessibility because you are limiting choices), but it gives you an idea of how the control looks.
All of the examples except the bottom two let you select from the dropdown or enter text yourself.
This is not the place to give you the technical solution on how to do this, but there are plugins for just about any web scripting frameworks (jQuery, MooTools, etc.) that can help you generate this. To get started, you can just search for "jquery combobox" in your preferred search engine provider (or whatever javascript framework you might be using).
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.