Hi there. It's not difficult to do in umbraco, though it depends on some javascript.

You start with hardcoding the select box, I havent found a method to extract the info with razor yet, so it can become dynamic.

select class="target">
    <option value="All">All</option>
    <option value="SEO">SEO</option>
    <option value="Marketing">Marketing</option>
    <option value="Website">Website</option>
    <option value="Ecommerce">Ecommerce</option>
    <option value="SEM">SEM</option>
    <option value="Apps">Apps</option>

</select>

Then with jquery you set a query in the URL like this ie:

<script>

    $(".target")
      .change(function () {
          var str = "";
          $("select option:selected").each(function () {
              str += $(this).text() + " ";
          });
          // CHANGE URL
          url = "?selected=" + str;
          window.location.href =url;
      })
</script>

Then you can make razorscript which reads the query and show selected items based on the query. I havent made the script yet, but I am working on a simple filter search like the one you are asking for, its just not finished, but what I have shown here is the sketch-up.

Here is an example on how to extract the query:

@{if (Request.QueryString["selected"] == "SEO")
{
// do something, show specific items
}
}

Here's some razor to extract the children with the property, all you need here is some IF sentence to find the value of projecttypes.

@{
  var selection = CurrentPage.Children();

  foreach (var item in selection)
  {
      if (item.HasValue("projecttypes"))
      {
      @item.projecttypes;
    <li><a href="@item.Url">@item.Name</a></li>

      }
  }
}