probably best is simply wordpress.org - zillions of "themes" (i.e. templates) and plugins (i.e. add-ons). Okay, maybe not zillions, but tens of thousands. Also has a very developed wiki with almost all the developer info you want.
Recently I created a test website and downloaded a number of free templates. I look at how easy they are to set to the various common design points: color, width, sidebars, etc. If I mess up the test site there is no problem to delete it and then start over :-)
Then I look at their paid versions to see what this step up offers. I've found quite a few good free and paid ones this way. Before this I focused too much on searching for the perfect one and not getting to know what's out there.