What are general methods to enable fields for input by visitors?

Drupal's many node types are great for making all kinds of content, and CCK multiplies the possibilities many times over.

One thing I can't quite grasp, though: Is there some general, built-in method for enabling individual fields that allow visitors to input data?

To clarify that, let me first note what I'm not talking about:

1) Not input of full nodes by users. That's all easy and understandable.
2) Not comments. That's a known and easy way to allow input by visitors, but I'm looking for actual fields.
3) Not addition of regular fields via CCK. Whatever the field type, CCK allows input into the field by the node author, yes, but not by other visitors.
4) Not (only) polls or ratings widgets. As I understand it, these allow multiple visitors to input data related to a node, which is on the right track, but such data is limited only to radio button-style selection from choices (whether poll choices or number of rating stars).

What I'm looking for is fields allowing flexible input of data. "Flexible" may or may not be restricted by user permissions, by length limitations, etc., but exact restrictions are a secondary concern for now.

With valid input, each node would have multiple values associated with it for that field (perfectly possible technically, I assume, as a node can have multiple comments associated with it). It would then be possible to manipulate those fields: list them, average them (if numbers), etc.

Examples of possible uses might best illustrate my wish:

a) Allow registered users to freely enter guesses in a contest. A site admin could look at the list of input guesses, and determine the winner.
b) Allow multiple instances of a ratings widget (such as three separate instances of the same ratings widget, each rating some different aspect of the node's content).
c) Allow polls or multiple-choice questions with open-end "Other" selections. (You can of course create "Other" as a poll selection, but I don't know any way to enable "Other (please describe)", followed by a field for input of the "please describe" response.

This is a follow-up to my question at http://drupal.org/node/191680 . That request targeted a couple of special situations involving ratings; here I'd like to ask more broadly about general methods for a wider range of uses.

Thanks in advance for help! This issue is important for a client's site, and I would think is of interest to many Drupal users. I've been scouring the forums and modules, without any clear solutions yet. I'll be happy to review and summarize any info received, in this thread and on my site.