I haven't found an English map on the web, but you can find a Chinese map of all the routes here: http://www.ttj.tw/en_route.html
They run every 10-15 minutes and are nicer than most of the pay busses I've been on. The bus drivers are friendlier too and will tell you where to get off if you can tell them or show them where you want to go. The bus does flash the next stop on a display at the front of the bus in both English and Chinese; some of them make announcements but I don't think those are bilingual.
I've found it to be a pretty slow way to travel if you're going a long distance on the weekend, because of so many people getting on and off. But during the week they're not crowded and as fast as a normal bus.
There's also an option on Google maps where you can search for your starting point and destination and get it mapped out by public transportation. I've found this really helpful if I need to go to a part of the city I'm not familiar with. If you can recognize street names and places in characters it's not a bad way to get around on occasion!