This doesn't exactly answer your question, but I don't think you'll find a single, centralised resource.
There's, of course, a reason behind why it's humanly possible to put together a resource like Seat 61. In many countries, trains are either nationalised (most countries) - either fully or like in the UK where the infrastructure is nationalised but the train operations aren't. Regardless of how the infrastructure is operated, train services are often either run by a single company / public body or an oligopoly of companies because of the capital involved and the limited nature of rail network 'resource'.
Bus companies in most countries on the other hand are more loosely regulated than trains or even other sectors such as airlines. These may still be a handful in Europe and North America, but the moment you get to Asia things get more complex as you'll literally find dozens of companies operating. Additionally, unlike train and airline reservation and scheduling systems which are largely computerised, bus services in many countries aren't at all making it harder to put together a centralised resource.
You'd be better off asking for bus schedule and information resources for individual countries or regions you're interested in.
EDIT: I recently came across this website called The Bus Station. It does not contain in-depth articles and guides as Seat61 does but for many countries it does have website / bus operator listings. Not the nicest site on your eyes though! Better than nothing I'd say if you can still at least look up bus schedules online. One thing to note could be that in parts of the world, bus operators offering online booking / schedules might be costlier ones and you may as well find cheaper operators once you're in the country.