Here's one such example:

public class loadingSpinnerCtrl {
    public void spin() {
        long now = datetime.now().gettime();
        while(datetime.now().gettime()-now<5000); // Busy loop for 5000 ms
    }
}

Page:

<apex:page controller="loadingSpinnerCtrl">
    <apex:form id="form">
        <apex:commandButton action="{!spin}" value="Show Spinner" rerender="form" status="status" />
        <apex:actionStatus id="status">
            <apex:facet name="start">
            <div style="position: fixed; top: 0; left: 0; right: 0; bottom: 0; opacity: 0.75; z-index: 1000; background-color: black;">
                &nbsp;
            </div>
            <div style="position: fixed; left: 0; top: 0; bottom: 0; right: 0; z-index: 1001; margin: 30% 50%">
                <img src="http://upload.wikimedia.org/wikipedia/commons/e/ed/Cursor_Windows_Vista.gif" />
            </div>
            </apex:facet>
        </apex:actionStatus>
    </apex:form>
</apex:page>

Feel free to adjust CSS as you like. Works in my developer org.

Here's one such example:

public class loadingSpinnerCtrl {
    public void spin() {
        long now = datetime.now().gettime();
        while(datetime.now().gettime()-now<5000); // Busy loop for 5000 ms
    }
}

Page:

<apex:page controller="loadingSpinnerCtrl">
    <apex:form id="form">
        <apex:commandButton action="{!spin}" value="Show Spinner" rerender="form" status="status" />
        <apex:actionStatus id="status">
            <apex:facet name="start">
            <div style="position: fixed; top: 0; left: 0; right: 0; bottom: 0; opacity: 0.75; z-index: 1000; background-color: black;">
                &nbsp;
            </div>
            <div style="position: fixed; left: 0; top: 0; bottom: 0; right: 0; z-index: 1001; margin: 30% 50%">
                <img src="http://upload.wikimedia.org/wikipedia/commons/e/ed/Cursor_Windows_Vista.gif" />
            </div>
            </apex:facet>
        </apex:actionStatus>
    </apex:form>
</apex:page>

Feel free to adjust CSS as you like. Works in my developer org.

Edit: Moved margin up by 20%. Now much closer to center of screen.