I recently came across some nice and easy to implement Ajax codes.
Swazz Javascript Calender
http://calendar.swazz.org/
Nice looking popup window to display a webpage / website
http://orangoo.com/labs/GreyBox/
Star rating bar allowing users to rate a webpage etc
http://www.masugadesign.com/the-lab/scripts/unobtrusive-ajax-star-rating-bar/
FrogJS Javascript Image Gallery
http://www.puidokas.com/portfolio/frogjs/
Enables draging of content on a webpage using an AJAX script
http://www.dhtmlgoodies.com/index.html?showDownload=true&whichScript=dragable-content
AJAX code for sorting tables automatically or manually by moving rows
http://friedcellcollective.net/js/SortedTable/index.html
Table row striping, column sorting, column resizing and cell editing
http://www.millstream.com.au/upload/code/tablekit/
Simple Ajax Chat Room
http://www.linuxuser.at/index.php?title=Most_Simple_Ajax_Chat_Ever
Ajax navigation system
http://javascript.internet.com/ajax/ajax-navigation.html
Username availability checker
http://javascriptsource.com/ajax/check-username-signup.html