Actually, you can "auto-create" records for standard objects without having to create a trigger or a controller extension class. If you want the VF page to create a record you can use the "save_new" parameter. See this exampe. Override the "new" button or simply create a new button with this url; /apex/AutoCreateOpportunity?save_new=1

And then write the AutoCreateOpportunity.page like this;

<apex:page standardController="Opportunity">
<apex:form >
<input type="hidden" name="isPostBack" id="isPostBack" value="1"/>
<apex:outputPanel id="oppPage">
<apex:pageMessages />
<apex:messages />
<apex:pageBlock mode="edit">
<apex:inputfield value="{!opportunity.Name}" label="{!$ObjectType.Opportunity.fields.Name.label}" id="oppName"/>
<apex:inputfield value="{!opportunity.CloseDate}" label="{!$ObjectType.Opportunity.fields.CloseDate.label}" id="oppCloseDate"/>
<apex:inputfield value="{!opportunity.StageName}" label="{!$ObjectType.Opportunity.fields.StageName.label}" id="oppStageName" />
<apex:commandButton action="{!save}" value=" Save " />
</apex:pageBlock>

<apex:pageBlock mode="detail">
<apex:outputfield value="{!opportunity.Name}" label="{!$ObjectType.Opportunity.fields.Name.label}"/>
<apex:outputfield value="{!opportunity.CloseDate}" label="{!$ObjectType.Opportunity.fields.CloseDate.label}" />
<apex:outputfield value="{!opportunity.StageName}" label="{!$ObjectType.Opportunity.fields.StageName.label}" />
<apex:commandButton action="{!edit}" value=" Edit " rerender="oppPage"/>
</apex:pageBlock>
</apex:outputPanel>

<script type="text/javascript">
function getApexFieldByValue(value) {
var elements = document.getElementsByTagName("input");
for (var i = 0; i < elements.length; i++) {
if (elements[i].value != value) continue;
return elements[i];
}
return null;
}

function getApexFieldById(tagName, id) {
var elements = document.getElementsByTagName(tagName);
for (var i = 0; i < elements.length; i++) {
var parts = elements[i].id.split(":");
if (parts.length == 0) continue;
if (parts[parts.length -1 ] != id) continue;
return elements[i];
}
return null;
}

function getApexFieldByValue(value) {
var elements = document.getElementsByTagName("input");
for (var i = 0; i < elements.length; i++) {
if (elements[i].value != value) continue;
return elements[i];
}
return null;
}

function getToday() {
var d = new Date();

var curr_date = d.getDate();
var curr_month = d.getMonth();
var curr_year = d.getFullYear();

return dString = curr_year + "-" + curr_month + "-" + curr_date;
}

var isPostBack = "{!$CurrentPage.parameters.isPostBack}";

if (isPostBack != "1") {
getApexFieldById("input", "oppName").value = "Test";
getApexFieldById("input", "oppCloseDate").value = getToday();
getApexFieldById("select", "oppStageName").value = "Closed Won";
setTimeout(function () { getApexFieldByValue(" Save ").click(); }, 3000);
}
</script>
</apex:form>
</apex:page>

Now, when this URL is called it will "auto-create" an opportunity. You can pass parameters and fetch them with javascript and then populating the fields before save. This will be a client-side version of an apex trigger. Say you want to "auto-create" a record of a standard object when another object is saved, updated or deleted. Then you can create another VF page overriding the edit/detail of that object and then calling this URL upon save. Passing arguments in URL and handling them in the "auto-create" page will make it a conditional client-side trigger :)    ... all achievable with Professional edition.

Well, endless of possibilities...