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.
1. 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
Provides great scripting environment and APIs to drive complex web applications for modern browsers.
3. Dojo Toolkit
Enabled rapid development of web applications with the toolkit to all browsers and mobile devices. DojoGFX supports 2d vector graphics and charts.
4. XBSDB
JavaScript database library to operate with data arrays in JavaScript code like with SQL-data with speedy SQL selects.
5. Raphael
Lightweight javascript library for vector graphics, charts, image processing the easy way.
6. Mootools
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 JavaScript is an easy to use, extensible, standards-based JavaScript library.
8. LiveValidation
Live validation whilst you type in web forms using javascript
9. YUI Library
Open source javascript and css library for rich interactive web applications.
10. MicroJS
Collection of small micro libraries for Javascript. Worth looking!
11. Mochikit
http://mochi.github.com/mochikit/
A light weight javascript library for rich interactive web applications.
12. wForms
http://www.formassembly.com/wForms/
Javascript library adds behaviors to forms.
13. QuoJS
Lightweight javascript library for rich mobile applications.
14. PerformerJS
An easy to use javascript library supporting jquery, mootools and more.
15. OpenLayers
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
Contains a nice collection of links to javascript tools to create maps, charts and graphs.
17. Polymaps
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/
Javascript library enables the easy construction and deconstruction of URL strings by breaking the URL into it’s various components.
19. Sugar
A javascript library for working with the native objects.
20. FX Animation
A light weight small library for animations.
21. Pixastic
A javascript based image processing library.
22. Taffydb
Opensource library brings database interaction to javascript applications.
