Form pick list/open text field

You should move the question to Products.  "Community" is for issues with the Marketo Nation (this website) itself -- and yes, this is confusing.

This is most easily done using Visibility Rules. Add a Rich Text area and set it to be visible only when the Select value is "Other."

I wrote about a more elegant approach here: http://blog.teknkl.com/forms-add-other-option-without-new-field/

Hi Sara -

Visibility Rules can do this easily for you. Marketo's guide is here:

Dynamically Toggle Visibility of a Form Field - Marketo Docs - Product Docs

Note, however, that you need to include two fields on your form to use this method -- the field with your picklist values, and an additional text area field.  You can then set the visibility rules for the open text field to Show If Picklist = Other.

EDIT: Also, +1 to Sanford's approach above -- from a data/instance hygiene perspective, it's always good practice to limit the amount of additional fields you create for one-off purposes like this.