In some countries, such as Turkey, Greece, Portugal and maybe Ireland, as well as in various parts of the east, long-distance buses are often the better and cheaper alternative. In many parts of these countries there is no train service anyway. Trains and buses are about even in some countries like Hungary, although service on particular routes is usually better on one or the other. The farther south you go, the more buses will be an option, or perhaps your only choice.

There are a few private intercountry bus options, such as the British Eurolines, found at http://www.nationalexpress.com/eurolines/home/hp.cfm, going from London to points all across the Continent. Long distance buses can take you from one city center to another, often for less than the train, but with more limited options.

For a Eurail-type option using a bus, look at Busabout Europe (used to be Eurobus) (http://www.busabout.com/), which offers three loops to follow around Europe on daily buses. There are many options and finding pricing is tough, but for 2006 one loop was $500, two loops $815, a six-stop flexipass was $405, and on and on. If a month-long party bus is what you're after, by the way, check out such bus tour operators as http://www.contiki.com/, who specialize in 18-35 year old tour bus groups. Most folks I talked to said it was fun the first week or so, but then, well...

Here is a site that pulls together all kinds of junk about buses in Europe: http://www.busstation.net/busstneuw.htm. Another resource site to Europe that has a bus information page is http://www.attitudetravel.com/coach/. You can find additional long-distance bus information on these and other sites.