There are a few ways I've seen of displaying field-like inputs on the web without storing them in the submitted document, but I think what I'm about to describe is the easiest way of all.

Basically you don't give the field a name, but give it an ID instead. For example:

<input id="ToDo_tmp" type="text" />

In the browser this looks and acts like a real field. However, when you come to submit the form it isn't counted as a "Successful Control" and its name/value isn't sent to the server.

These kind of fields are really useful sometimes. As a simple example take the ToDo list form from the other week. As I mentioned then, the code added the given value to a set of hidden fields, all with the same name, which can be saved in a multi-value field.

Imagine we had a real field on our form called "ToDo" with "type="\hidden\"" in its HTML Attributes. This field is multi-value and separated by semi-colons. Using a clickable button, JavaScript and our temporary field we can add its values like so:

oFrm["ToDo"].value+=(document.getElementById("ToDo_tmp").value)+";"

Or, for you Prototype fans:

$("ToDo").value+=$F("ToDo_tmp")+";"

Obviously not a great example, but you get the idea, right? To add the temporary field you can use PassThru HTML and you can even give it a value using Computed Text. Note that this applies to all field types. Not just plain text ones.