Spokane Transit (Washington USA) has a new network map out that is one of the clearest I've ever seen. It carefully delineates not just frequent services from infrequent ones, but also presents cases where basic infrequent lines combine to form frequent segments, and ensures that peak commuter express services are visible but can't distract from the clear all-day pattern. The whole thing in its full glory is here: Download Spokane 2011 map. The legend, too, is both clear and wonkish at the same time.
Full disclosure: I was the lead planner on a restructuring study for Spokane Transit back around the turn of the century, and if I remember right, our project invented the continuous two-way frequent loop of Lines 33 and 44. (The loop is closed on the west side as Line 20, as shown on the full PDF.) Despite many excellent improvements (and some sad service cuts) since then, it's great to see it still operating.
This kind of two-way loop is often useful as a way to combine radial lines and grid elements into a single service. Line 33, for example, intersects Line s 24, 25, and 90 in a grid manner, one line north-south and the other east-west, allowing for a range of L-shaped trips via a connection at this point. However, Line 33 also flow through so that the same segments can also be experienced as radial; if you stay on the bus, you'll get downtown eventually, and to a lot of other useful destinations.
I sometimes caution against excessive attachment to loops. In some contexts, with far more financial resources, I might applaud the breaking up of this loop, as I did of London's Circle Line. Given the extreme financial pressure on US transit agencies, though, I would contend that Spokane's frequent loop was an efficient solution, maybe even an elegant one.
UPDATE: The next post on the Spokane map, looking at colorblindness issues and comparing the map to Portland's, is here.