Earlier today, Nick Jenkins filed a bug complaining about how cumbersome it is to just focus on one list (or calendar). And he's right. You basically have two choices: 1) go through the sidebar and manually un-check everything you don't want to see, which is ridiculously cumbersome, or 2) right-click on the item you do want to see and select "Show Only This whatever" from the pop-up menu, which check marks that item and clears all others.
Handy, right? Well, not so much if you use it frequently enough. Nick was asking for an easier way, so I suggested we make triple-clicking on a sidebar item a shortcut for the "show only" menu item. (Triple instead of double-click so it's not so easy to trigger by accident.)
Took about a half hour to hack together and I have to say I'm startled at how much easier this simple enhancement allows me to manage my task lists! I like it enough that I snuck it in at the last minute for Evolution 2.30.