I never hesitated to use javascript and ajax libraries for sleek user interactivity in web application.  Javascript run on client side and therefore does not put loads on server. You can find below some nice fast loading and lightweight libraries which you might find very interesting to learn and use it in your website.

js

1. jQuery

http://www.jquery.com

jquery

JQuery is the most popular powerful javascript library for php /ajax driven applications. A lightweight library is also possible for basic features. The autocomplete or suggestions is the most interesting feature in jquery, accomplished by using this plugin: http://jqueryui.com/autocomplete/

2. Prototype

http://prototypejs.org

pro

Provides great scripting environment and APIs to drive complex web applications for modern browsers.

3. Dojo Toolkit

http://dojotoolkit.org

dojo

Enabled rapid development of web applications with the toolkit to all browsers and mobile devices. DojoGFX supports 2d vector graphics and charts.

4. XBSDB

http://xbsdb.org

xbsdb

JavaScript database library to operate with data arrays in JavaScript code like with SQL-data with speedy SQL selects.

5. Raphael

http://raphaeljs.com

raphael

Lightweight javascript library for vector graphics, charts, image processing the easy way.

6. Mootools

http://mootools.net

moo

MooTools is a compact, modular, open source JavaScript framework designed for JavaScript developers. Write powerful, flexible, and cross-browser code with its elegant, well documented APIs. Autocompleter extension is also great for auto suggestions. http://digitarald.de/project/autocompleter/

7. JellyJS

http://code.google.com/p/jelly-javascript/

jelly

Jelly JavaScript is an easy to use, extensible, standards-based JavaScript library.

8. LiveValidation

http://livevalidation.com

vald

Live validation whilst you type in web forms using javascript

9. YUI Library

http://yuilibrary.com/

yui

Open source javascript and css library for rich interactive web applications.

10. MicroJS

http://microjs.com

microjs

Collection of small micro libraries for Javascript. Worth looking!

11. Mochikit

http://mochi.github.com/mochikit/

mochikit

A light weight javascript library for rich interactive web applications.

12. wForms

http://www.formassembly.com/wForms/

qdoema

Javascript library adds behaviors to forms.

13. QuoJS

http://quojs.tapquo.com

quo

Lightweight javascript library for rich mobile applications.

14. PerformerJS

http://performerjs.org

performer

An easy to use javascript library supporting jquery, mootools and more.

15. OpenLayers

http://openlayers.org

opwnlYWE

OpenLayers makes it easy to put a dynamic map in any web page. It can display map tiles and markers loaded from any source.

16. Data visualisation

http://selection.datavisualization.ch

dv

Contains a nice collection of links to javascript tools to create maps, charts and graphs.

17. Polymaps

http://polymaps.org

poly

Polymaps is a free JavaScript library for making dynamic, interactive maps in modern web browsers.

18. URL Library

http://www.fliquidstudios.com/projects/javascript-url-library/

url

Javascript library enables the easy construction and deconstruction of URL strings by breaking the URL into it’s various components.

19. Sugar

http://sugarjs.com

sugar

A javascript library for working with the native objects.

20. FX Animation

http://fx.inetcat.com

fx

A light weight small library for animations.

21. Pixastic

http://www.pixastic.com

pix

A javascript based image processing library.

22. Taffydb

http://www.taffydb.com

taffy

Opensource library brings database interaction to javascript applications.