A spam prevention tool. Also see Mollom and Honeypot.

Here are some examples of using Drupal's states API. It shows the use of OR and XOR (which only work with jQuery 1.8 or above, apparently). More examples can be found in the examples project in the form example module.

Here's a Tampermonkey script which adds a Save and Edit button to the mini panels content edit page for every site :) If you don't have the button, then every time you save, you are brought back to the mini-panels content overview page, which gets kind of long.

Webform Remote Post is a module that works along the Webform module. It eases the integration between Webforms and other web applications (including systems like Salesforce and Eloqua).

I can't believe that I haven't heard of this module until now. Maintained by Joachim Noreiko, a very prolific module author, this module lets you create views where the values can be edited inline.

Joachim's demo vid (~5 minutes) https://www.youtube.com/watch?v=g_D4z4Bw6iw

Before using this module, check out Entityforms instead, which I believe will overtake Webform. 

There are contributed modules that will help with this that are listed below. You can also use Form API's #attributes to add in an HTML5 placeholder. Here's an example using Drupal Form API:

A nice way to add placeholders to input fields. "This module auto-converts form input fields with space-hogging labels into compact mobile-friendly input fields, with placeholders that animate into much smaller labels the moment text is keyed in. "

"Maxlength allows you to set maximum length of any field on any form making use of the form API. This includes fields created using the UI. (Example: any text field on a node)

From the project page: