Trello is super useful and I love it. These are some web extensions I made. I use them almost every day so they should be kept up-to-date as Trello changes how things work. They work in Chrome, Firefox and Opera.
Sets completed checklist items to be hidden by default when you open a card. This is useful if you don’t complete items in order, since everything that’s left still to do is nicely grouped together. (You can show them again with the link in the checklist header.)
This dims most of your lists by default, and highlights others based on the list title.
*Other lists are dimmed only when your board has a list with one of these titles.