Multi select categories in magento

General information for the website: Multi select categories on magento
Kind of development: Customization of existing website
Description of every page/module: On magento site need subcategories to be multiple select.
so user can select more than one categories from left bar.
for example these sub categories.
Party Dresses (38)
Evening Dresses (25)
Mini Dresses (24)
user must be able to select party dresses and evening dresses while keeping the existing filters coming from colors and sizes.
Description of requirements/features: 1.Third level categories on left must be multi selectables.
while keeping the rest of filters active.

3. first and second level categories must have always same seo url as on top menu and left menu.
and don't care for third level category url
Payment methods: Paypal
Extra notes: