Forms API is great, let me start there. However there are some cases where you want to ... ahem, go around it. One such cases is with checkboxes (where you have a number of checkboxes), specifically when you want to alter just one of the set. Here's how to fix that!
One of the great usability features in the Drupal Form API are simple validators, such as #required. Just set it to true, and Drupal takes care of it. However, when in a multistep scenario, simple validators go haywire since moving to a different step validates the form. Normally, you cannot override form errors within your validation hook... but there's a workaround.
More often than not, you'll come to a point where you need a field value of a node. Not the whole node, just a field. The most common case would appear to be when you need an image (say a promotional image). Some ways are easier than others for it, however you need to think of what you are doing under the hood.
I assume that if you've build a relatively large site, you've stumbled on Nodequeue by now ( summary: it allows you to create lists of arbitrarily ordered nodes ). But clients always had a thing for imagining things that are normally tricky to do, and one of them is to create a block that has arbitrarily ordered content per section of the site, usually by taxonomy term.
What every new developer in Drupal needs to get to his head (and hears all the time in the community), is that no matter what, you should never EVER hack core. Except when your problem is one that has absolutely no other solution. In our case, using an alternate administration theme, will cause access errors to administration pages to be rendered in that theme -- and that cannot be solved otherwise.
A client needed the ability to easily zoom into images in place. That is, display a small image, and when hovering, zoom a part of the image. In that spirit, I found the freely available cloud zoom library. Though I did find a Drupal module for that, it only exposed a very limited amount of options that are available, and the dev version, which included more features, had bugs (hence the dev). This is my implementation, which (hopefully) will help people out.