To show current Data and Time is a most popular design tool. I use own simpliest ones.

Backgroundscroller scrolls background text; very nice trick.

Background Color Wizard helps to check on compatibility of background and text colors, try bgwizard.

Moving Dropdown Menu helps to create Jump-forms in your site without CGI, try DropDown Menu.

Back and Forward Buttons help you reload pages.

Random phrase or image can be inserted in any place where you want. You add your phrases and images (banners, for example).

Visible - Unvisible text or image - try and see.

Countdown in status string is used often in any online tests, games...

Watermark is used as shopping cart in online stores (of course, object is not picture).

World Time Ticker in the Titlebar is an interesting and effect idea.

ScrollUp Text 1 and ScrollUp Text 2 are good for short headlines of the news.

5-function calculator (add, substract, multiply, divide, power) is useful if you need to have calculations on site.

All above is free for all. Of course, there are much more JS public libraries as JavaScripts.com, TUCOWS and CNet. Interactive course of JS you can take in IT Knowledge.com.

JavaScript can be used not for only client-side programming but also for server-side programming. Server-side JavaScript commands are also embedded within the HTML making up your Web pages, but they are processed on the server. Server-side JavaScript can then generate and send HTML, incorporating the results of this processing for display on the reader’s browser. In other words, your Web pages’ responses to actions performed and information provided by their readers take place on the server as opposed to the reader’s computers. Client-side and server-side JavaScript can be mixed within the same Web page. Server-side processing allows users to respond in ways not possible using just client-side JavaScript. For example, JavaScript running on a server can read from and write to files as well as accessing databases. Your Web pages can give their readers access to information stored on such files and databases, or they can respond to readers using information stored on the server from previous visits. Information can be shared among visitors to your Web page and between different Web pages. Server-side JavaScript requires the use of Netscape’s server software. Netscape’s Dynamic HTML and Netscape's Visual JavaScript are based on JavaScript.