I'm using the Google Fusion (beta) again. Here we see every MBTA transit stop represented as a marker on the map. I'm using a color scheme where blue represented stops that only average less than one vehicle every 15 minutes (cold), purple is at least every 7.5 minutes, yellow is at least every 5 minutes, and red is better than that (hot!). I'm assuming a 20 hour span of service, so rush hour-only buses do not count very heavily. You can click on a marker to find out what routes stop there, and how many times in a typical weekday.
It should be easy to pick out some of the corridors: Comm Ave, Mass Ave, Washington St (several of them), Blue Hill Ave, etc. The hottest spot is Dudley, as it should be, followed by Forest Hills, Ashmont, and Sullivan.
I was inspired to check out this data while standing around, earlier this week, in Somerville Union Square and realizing that there was not going to be any bus coming for at least 25 minutes -- even though many bus lines connect there. Sure enough, that part of Somerville is mostly blue, except at the very center where the infrequent and rush hour-only buses happen to add up to a yellow dot.
Anyway, I'm still learning how to use the Google Fusion data visualization tool, and hopefully I can come up with some better maps in the future.