The enhanced ECT Side Filter Bar 2 has all the great features of the regular side filter bar but with many extras including checkboxes allowing the possibility to make more than one selection per group in order to add to the items that are displayed. When making selections you can choose to keep all other attributes (allowing you to extend selections) or remove attributes with zero products. Hidden attributes in long lists can be replaced with a button/link/image to display more.
The side filter has also had more of the style attributes added to the ectcart.css style sheet, along with several optimizations and so produces more efficient code. It can work in tandem with the regular top filter bar so you can have different filters at the top and side if you choose to. It requires a minimum version of ECT v6.9.3... more details