Make sure you implement the yahoo and connection libraries.
You can download these from the YUI developer network.

<script type="text/javascript" src="static/js/yui/yahoo-min.js"></script>
<script type="text/javascript" src="static/js/yui/connection-min.js"></script>

The script:

<script type="text/javascript">
      var request;
      function doSomeRequest(servletName, servletArguments){
          var arg = "myParam=" + getPopupContent(servletArguments).replace(/[\n\r\%]/g,''); //I replaced linebreaks and % which breaks requests.
          YAHOO.util.Connect.asyncRequest('POST', servletName, callback, arg); //note the arg is only for POST methods
          	var callback =
			{
			  success: function(o) {
			  	//do something with the response
			  },
			  failure: function(o) {
			  	//do something with the error
			  }
			}
		}
</script>

Download the files or see more examples on YUI Developer network.

Javascript frameworks are for easing development and dynamic web apps.
These toolkits enables you to deal with Ajax calls in an easy and fun way and it’s also cross browser safe.

It can be handy, if you know already something about JSON:
JSON (JavaScript Object Notation) is a lightweight data-interchange format. It is easy for humans to read and write and it is easy for machines to parse and generate. It is based on a subset of the JavaScript Programming Language, Standard ECMA-262 3rd Edition – December 1999.
JSON is a text format that is completely language independent but uses conventions that are familiar to programmers of the C-family of languages, including C, C++, C#, Java, JavaScript, Perl, Python, and many others. These properties make JSON an ideal data-interchange language.

It’s a personal choice which of the Javascript Frameworks you should choose.
Basically they are all more or less providing you the same functionality, and they all
contain excellent documentation and API’s with examples.
I will show you the 6 most popular frameworks, and tell you the current version,
download link and which websites these frameworks use. Also the size of the .js file,
which is an important detail.

* = compressed gzipped / uncompressed

These handy Salesforce functions I used a lot, for doing Salesforce database queries.
You can use these functions in your S-Controls:

/**
 * Get an array with all the possible fields names from a object
 * @param objectType object to get the fields from.
 * @return array with all the fieldnames of the given object
 */
function getFields(objectType) {
	var describeSObjectResult, fields, i;

	describeSObjectResult = sforce.connection.describeSObject(objectType);
	fields = [];
	var length = describeSObjectResult.fields.length;
	for (i = 0; i < length; i++)
	fields[i] = describeSObjectResult.fields[i].name;

	return fields;
}

/**
* Result handler, forwards you to the detail page,
* if there is an error give alert message.
* @param result - can be a (sf query) object, or a textmessage string.
* @param idVal - the id of the detailpage to forward to.
*/
function resultHandler(result, idVal){
	var msg = "";

	if (typeof(result) == "object") {
		if (result[0].getBoolean("success")) {
			msg = "";
		} else {
			msg = "Error: " + result[0];
		}
	} else {
		msg = result;
	}
	if (msg != ""){
		alert(msg);
	}
	self.location.href = "/" + idVal;
}

Oh I loved my previous project, where I had to work with the CRM: Salesforce.
Salesforce coding was fun: S-Controls (iframes), Salesforce Ajax (Javascript with Salesforce Query Language) and Apex (Java like Salesforce serverside coding).

These are handy Salesforce date/time functions which you can use in your S-Controls:

/**
* Return the current time
* @return javascript date time object
*/
function getCurrentSalesForceTime(){
	var currentDateTime = sforce.connection.getServerTimestamp();
	currentDateTime = setStringToDateTime(currentDateTime.timestamp);

	return currentDateTime;
}

/**
* Set Javascript Date Time to Salesforce Date Time;
* @param dateTimeObj - date time string
* @return salesforce date time object
**/
function setStringToDateTime(dateTimeObj){
	var dateTimeObj;

	if (dateTimeObj != null) {
		dateTimeObj = sforce.internal.stringToDateTime(dateTimeObj);
	}

	return dateTimeObj;
}

/**
* Set Salesforce Date Time Object to Javascript Date Time
* @param dateTimeObj - Salesforce date time object
* @return string date time
**/
function setDateTimeToString(dateTimeObj){
	var dateTimeObj;

	if (dateTimeObj != null) {
		dateTimeObj = sforce.internal.dateTimeToString(dateTimeObj);
	}

	return dateTimeObj;
}