Traveling by Bus:

Buses are generally less comfortable than trains, but can be considerably cheaper. Unless you're looking for a bus tour, your best to figuring out buses is to ask the people at your hostel about buses. Many cities have both local/regional bus stations, as well as long-distance bus lines

European Bus Lines:

Eurolines -- Eurolines offers both passes and single tickets to destinations across Europe.

About.com's Europe for Visitors Guide -- About.com has more info and links to a number of European bus lines

Bus Tours:

One option you might consider is taking a bus tour. A number of different bus tours exist across Europe.

Bus Tours

Busabout -- Busabout offers a hop-on hop-off travel network around Europe covering 30 destinations and 10 countries. Designed with backpackers and independent travellers in mind, the bus network runs daily from May until October and offers multiple pass options.

Hostelworld Tour -- Hostelworld offers a number of tours. Search by country to see what they have availible.