A couple of helpers...

filterBy.js

function filterBy(selector) {
	var c = selector.value, loc = document.location, q = loc.search;
	q = addOrReplaceQueryStringParam(q, "category", c);
  	document.location = loc.pathname + q;
}

function gotoCategory(selector) {
	var c = selector.value;
	document.location = "/produkter?category=" + encodeURIComponent(c);
}

function addOrReplaceQueryStringParam(qs, name, value) {
    qs = qs.indexOf("?") == 0 ? qs.substr(1) : qs;
    var params = qs ? qs.split("&") : [ ];
    if (qs.indexOf(name) >= 0) {
        for (var i = params.length - 1; i >= 0; i--) {
            if (params[i].indexOf(name) == 0) {
                params[i] = name + "=" + encodeURIComponent(value);
            }
        }
    } else {
        params.push(name + "=" + encodeURIComponent(value));
    }
    return "?" + params.join("&");
}