Lightbox
Lightbox is the de-facto standard image popup and image gallery script, lots of variants have been produced. ajax script example here ajax script demo here ajax script download here
Sudoku
This one is for all the sudoku addicts out there. ajax script example here ajax script demo here ajax script download here
Ajax Captcha
This is the coolest captcha i have come across to date, i really get wound up with the text captchas, they seem to be getting harder to decode, anyhow we have here a nice visual captcha that works be dragging and dropping the requested image onto the the hot spot ajax script example here ajax [...]