The excellent Underscore.js has a throttle function. You pass in the handler that you want to throttle and get back a rate-limited version of the same function.

var throttled = _.throttle(someHandler, 100);
$(div).click(throttled);

http://documentcloud.github.com/underscore/#throttle

Here's a simplified version that I've used in my own code:

function throttle(func, wait) {
    var timeout;
    return function() {
        var context = this, args = arguments;
        if (!timeout) {
            // the first time the event fires, we setup a timer, which 
            // is used as a guard to block subsequent calls; once the 
            // timer's handler fires, we reset it and create a new one
            timeout = setTimeout(function() {
                timeout = null;
                func.apply(context, args);
            }, wait);
        }
    }
}

A good way to test it is by firing off a bunch of scroll events and watching your handler log to the Firebug console:

document.addEventListener("scroll", throttle(function() {
    console.log("test");
}, 2000), false); 

Here's a version that limits click-events on divs to once every 30 seconds, as requested (requires jQuery):

$("div").click(throttle(function() {
    // ajax here
}, 30000));

If you want to rate limit, then unfortunately the _.throttle method that underscore.js provides is not your solution. Throttle will simply ensure your method is never called more than X seconds, and therefore all subsequent function calls will be disregarded until that period has passed.

If you want to rate limit so that you never call your function more than X times per second, but don't lose those function calls altogether, then you need a wholly different solution.

I have written an underscore extension at https://gist.github.com/1084831

You can see a working example at http://jsbin.com/upadif/8/edit#preview

Create a boolean canFireRequest, or whatever, flag and set it to false after each ajax request. Then create a 30 second time span that sets it back to true; check the flag's value before each new request.

Here's a rough example:

if ($(this).data('canFireRequest')) {
    // Ajax request goes here
    $(this).data('canFireRequest', false);
}

setTimeout(function() {
    $(this).data('canFireRequest', true)
}, 30000);