Great set of beautiful and useful Mootools plugins. Accordion Image Menu, Rounded Corners, Slideshow, Lightbox, Multibox, Sortable Table, Slider ...
A great tutorial on how to create sliding and draggable actions menu with jQuery, that appears when checkboxes are selected.
A free web based application to adjust browser size automatically from iPhone-sized browser to a MacBook Pro sized browser window.
An open source application for displaying the availability calendar of whatever you might need it, apartments, holiday cottage, hotel rooms etc.