bootswatcherizr vanilla javascript and a bonus coffeescript flavour (converted to coffee with js2coffee) version. Project is currently alpha [not meant for serious use]

bootswatchrizr.js

(function(e, a, g, h, f, c, b, d) {
  if (!(f = e.jQuery) || g > f.fn.jquery || h(f)) {
    c = a.createElement("script");
    c.type = "text/javascript";
    c.src = "http://ajax.googleapis.com/ajax/libs/jquery/" + g + "/jquery.min.js";
    c.onload = c.onreadystatechange = function() {
      if (!b && (!(d = this.readyState) || d === "loaded" || d === "complete")) {
        h((f = e.jQuery).noConflict(1), b = 1);
        return f(c).remove();
      }
    };
    return a.documentElement.childNodes[0].appendChild(c);
  }
})(window, document, "1.3.2", function($, L) {
  var $e, l, r;
  $e = void 0;
  l = void 0;
  r = void 0;
  $e = void 0;
  l = void 0;
  r = void 0;
  if ($(".bootswatcher")[0]) {
    return $(".bootswatcher").remove();
  } else {
    $e = $("<select class=\"bootswatcher\"><option>Amelia</option><option>Cerulean</option><option>Cosmo</option><option>Cyborg</option><option>Journal</option><option>Readable</option><option>Simplex</option><option>Slate</option><option>Spacelab</option><option>Spruce</option><option>Superhero</option><option>United</option></select>");
    l = 1 + Math.floor(Math.random() * $e.children().length);
    r = "<link rel=\"stylesheet\" href=\"http://bootswatch.com/default/bootstrap-responsive.min.css\">";
    $e.css({
      "z-index": "99999",
      position: "fixed",
      top: "5px",
      right: "5px",
      opacity: "0.5"
    }).hover(function() {
      return $(this).css("opacity", "1");
    }, function() {
      return $(this).css("opacity", "0.5");
    }).change(function() {
      if (!$("link.bootswatcher")[0]) {
        $("head").append("<link rel=\"stylesheet\" class=\"bootswatcher\">" + r);
      }
      return $("link.bootswatcher").attr("href", "http://bootswatch.com/" + $(this).find(":selected").text().toLowerCase() + "/bootstrap.min.css");
    }).find("option:nth-child(" + l + ")").attr("selected", "selected").end().trigger("change");
    return $("body").append($e);
  }
});

bootswatchrizr_coffee.js

((e, a, g, h, f, c, b, d) ->
  if not (f = e.jQuery) or g > f.fn.jquery or h(f)
    c = a.createElement("script")
    c.type = "text/javascript"
    c.src = "http://ajax.googleapis.com/ajax/libs/jquery/" + g + "/jquery.min.js"
    c.onload = c.onreadystatechange = ->
      if not b and (not (d = @readyState) or d is "loaded" or d is "complete")
        h (f = e.jQuery).noConflict(1), b = 1
        f(c).remove()

    a.documentElement.childNodes[0].appendChild c
) window, document, "1.3.2", ($, L) ->
  $e = undefined
  l = undefined
  r = undefined
  $e = undefined
  l = undefined
  r = undefined
  $e = undefined
  l = undefined
  r = undefined
  if $(".bootswatcher")[0]
    $(".bootswatcher").remove()
  else
    $e = $("<select class=\"bootswatcher\"><option>Amelia</option><option>Cerulean</option><option>Cosmo</option><option>Cyborg</option><option>Journal</option><option>Readable</option><option>Simplex</option><option>Slate</option><option>Spacelab</option><option>Spruce</option><option>Superhero</option><option>United</option></select>")
    l = 1 + Math.floor(Math.random() * $e.children().length)
    r = "<link rel=\"stylesheet\" href=\"http://bootswatch.com/default/bootstrap-responsive.min.css\">"
    $e.css(
      "z-index": "99999"
      position: "fixed"
      top: "5px"
      right: "5px"
      opacity: "0.5"
    ).hover(->
      $(this).css "opacity", "1"
    , ->
      $(this).css "opacity", "0.5"
    ).change(->
      $("head").append "<link rel=\"stylesheet\" class=\"bootswatcher\">" + r  unless $("link.bootswatcher")[0]
      $("link.bootswatcher").attr "href", "http://bootswatch.com/" + $(this).find(":selected").text().toLowerCase() + "/bootstrap.min.css"
    ).find("option:nth-child(" + l + ")").attr("selected", "selected").end().trigger "change"
    $("body").append $e