Here are some plugins I think will come in handy:

Jquery Validation
This plugin will check if all mandatory fields were filled. If not it will print a red "please fill in" message.

Dynatree
Ever wanted to build a list of folders, that have folders inside them? Since you are a PHP coder you know how much trouble it would be to acomplish this in PHP. With this handy plugin users can open folders without having to reload on each action. It also supports ajax, so when a user clicks on a folder, it will load only that portion.

Easy tooltips
Tooltips are great for providing more information to end users before they click a link. With this handy plugin you can even put html code into a tooltip!

Form Masks
You have a form and you want users to fill it a certain way (for ex: 12/32-365). Now you can coach users through each element of a form with masks. Look at these examples to better understand.

Carousel
We have all seen carousels on popular sites, such as bbc or cnn. In the old days these were acomplished with flash but it was very hard to pull information from a database, especially using xoops. With this plugin you can easily acomplish the same effect. It even allows ajax calls.

Let me know if this was usefull and I'll write some more.

Cool stuff. good one. and its really appreciated. I intend to put a few XOOPS themes on steroids! wait till i'm through.

Dynatree looks nice!
I started to use the TreeView Plugin. And now I have the dilemma of what will be better;)