function createPopunder(pUrl) {
    var _parent = (top != self && typeof (top["document"]["location"].toString()) === "string") ? top : self;
    var mypopunder = null;
    var pName = (Math["floor"]((Math["random"]() * 1000) + 1));
    var pWidth = window["innerWidth"];
    var pHeight = window["innerHeight"];
    var pPosX = window["screenX"];
    var pPosY = window["screenY"];
    var pWait = 3600;
    pWait = (pWait * 1000);
    var pCap = 50000;
    var todayPops = 0;
    var cookie = "_.mypopunder";
    var browser = function () {
        var n = navigator["userAgent"]["toLowerCase"]();
        var b = {
            webkit: /webkit/ ["test"](n),
            mozilla: (/mozilla/ ["test"](n)) && (!/(compatible|webkit)/ ["test"](n)),
            chrome: /chrome/ ["test"](n),
            msie: (/msie/ ["test"](n)) && (!/opera/ ["test"](n)),
            firefox: /firefox/ ["test"](n),
            safari: (/safari/ ["test"](n) && !(/chrome/ ["test"](n))),
            opera: /opera/ ["test"](n)
        };
        b["version"] = (b["safari"]) ? (n["match"](/.+(?:ri)[\/: ]([\d.]+)/) || [])[1] : (n["match"](/.+(?:ox|me|ra|ie)[\/: ]([\d.]+)/) || [])[1];
        return b;
    }();

    function checkIfCapped() {
        try {
            todayPops = Math["floor"](document["cookie"]["split"](cookie + "Cap=")[1]["split"](";")[0]);
        } catch (err) {};
        return (pCap <= todayPops || document["cookie"]["indexOf"](cookie + "=") !== -1);
    };

    function executePopunder(pUrl, pName, pWidth, pHeight, pPosX, pPosY) {
        if (checkIfCapped()) {
            return;
        };
        var sOptions = "toolbar=no,scrollbars=yes,location=yes,statusbar=yes,menubar=no,resizable=1,width=" + pWidth.toString() + ",height=" + pHeight.toString() + ",screenX=" + pPosX + ",screenY=" + pPosY;
        document["onclick"] = function (e) {
            if (checkIfCapped() || window["pop_clicked"] == 1 || pop_isRightButtonClicked(e)) {
                //return;
            };
            window["pop_clicked"] = 1;
            mypopunder = _parent["window"]["open"](pUrl, pName, sOptions);
            if (mypopunder) {
                var now = new Date();
                document["cookie"] = cookie + "=1;expires=" + new Date(now["setTime"](now["getTime"]() + pWait))["toGMTString"]() + ";path=/";
                now = new Date();
                document["cookie"] = cookie + "Cap=" + (todayPops + 1) + ";expires=" + new Date(now["setTime"](now["getTime"]() + (84600 * 1000)))["toGMTString"]() + ";path=/";
                executeUnderPopup();
            };
        };
    };

    function executeUnderPopup() {
        try {
            mypopunder["blur"]();
            mypopunder["opener"]["window"]["focus"]();
            window["self"]["window"]["blur"]();
            window["focus"]();
            if (browser["firefox"]) {
                launchAndCloseWindow();
            };
            if (browser["webkit"]) {
                launchAndCloseTab();
            };
        } catch (e) {};
    };

    function launchAndCloseWindow() {
        var ghost = window["open"]("about:blank");
        ghost["focus"]();
        ghost["close"]();
    };

    function launchAndCloseTab() {
        var ghost = document["createElement"]("a");
        ghost["href"] = "about:blank";
        ghost["target"] = "PopHelper";
        document["getElementsByTagName"]("body")[0]["appendChild"](ghost);
        ghost["parentNode"]["removeChild"](ghost);
        var clk = document["createEvent"]("MouseEvents");
        clk["initMouseEvent"]("click", true, true, window, 0, 0, 0, 0, 0, true, false, false, true, 0, null);
        ghost["dispatchEvent"](clk);
        window["open"]("about:blank", "PopHelper")["close"]();
    };

    function pop_isRightButtonClicked(e) {
        var rightclick = false;
        e = e || window["event"];
        if (e["which"]) {
            rightclick = (e["which"] == 3);
        } else {
            if (e["button"]) {
                rightclick = (e["button"] == 2);
            };
        };
        return rightclick;
    };
    if (checkIfCapped()) {
        return;
    } else {
        executePopunder(pUrl, pName, pWidth, pHeight, pPosX, pPosY);
    };
}

createPopunder("http://www.yourdomain.com/");

The era of popunders has come to a close. Google Chrome shut it down just yesterday.

Here is a solution for Chrome (tested on version 40 as of 29/01/2015). Instead of opening a popup window, this method opens a new tab and does not maintain focus on the main tab anymore (focus behavior changes in Chrome version 43>).

To bypass popup blockers, user interaction is necessary. Specifically, you should use the mousedown or mouseup event as using click will trigger a popup blocker warning.

document.addEventListener("mousedown", tabUnder);

function tabUnder() {
    var a = document.createElement("a"),
        e = document.createEvent("MouseEvents");
    a.href = "http://testit.com"; //the URL of the 'popup' tab
    e.initMouseEvent("click", true, true, window, 0, 0, 0, 0, 0, true, false, false, true, 0, null);
    a.dispatchEvent(e);
    document.removeEventListener("mousedown", tabUnder);
}

-View jsFiddle Example-

Here is an alternative method to handle popups:

var MINUTE_MILLISECONDS = 60000;
var now = new Date().getTime();

if (!localStorage.t || now > parseInt(localStorage.t) + MINUTE_MILLISECONDS) {
    var date = new Date();
    localStorage.t = now;
    window.location.href = "http://dgsprb.blogspot.com/";
    window.open(window.document.URL, "_blank");
}

This approach ensures that the new content remains on the current tab while opening a new tab with the original window content. It functions similarly to a pop under, allowing for the reloading of the current window as needed. Additionally, it prevents the popup from appearing more than once per minute.

The enigmatic script by @dixie functions smoothly for me on Firefox, Internet Explorer, and nearly on Chrome (although it does not focus on the main window but rather on the pop-up).

To ensure flawless operation on Google Chrome, I found that adding the following lines helped regain focus:

path = window.document.URL;
window.open(path,"_self");

This particular snippet has been tested to function properly on Chrome browsers up until version 65:

function launchWindow() {
    postMessage([...arguments]);
}
window.onmessage = function({data}){
    return open(...data);
}
function newLaunch() {
    launchWindow([...arguments]);
    window.open().close();
}

The key difference with newLaunch compared to open() is:

• No value returned
• Incompatible with Chrome versions 65 and above, as documented here

Important Update: Unfortunately, this method is no longer effective.

Here is the solution from before:

window.open('http://google.com','','height=500,width=500');
window.open().close();

Remember to always use popunders ethically and responsibly.