For some routes (or environmental concerns), it is a good idea to travel by train or by (intercity) bus instead of flying.
To find out what is worthwhile financially and in terms of time, GoEuro is ideally suited. There you can compare bus, train, carpool and flight, but only for “exact” routes (city to city) and with a fixed date. But then GoEuro indeed shows very clearly the differentoptions. Carpools (BlaBlaCar) are listed with the buses. The largest databases are available for Germany, Spain, the UK, Italy, the Netherlands, Belgium, Luxembourg and Switzerland. There are some filter and sorting options.
An even more comprehensive search provides Rome2rio. It compares everything from train rides to service buses, intercity buses, ferries, own car travel, carpooling and flights from and to different airports and also combinations of the aforementioned means of transport. The advantage as well as disadvantage at the same time of Rome2rio is that it does not take time data into account, but rather displays what the options usually cost, resulting in huge potential price margins. So that in order to search for a route for a certain day, Rome2rio is not that suitable. Instead, it’s good for finding out about the various available travel possibilities.
A lot of railway companies have some very cheap offers such as the saver fare Europe from the German “Deutsche Bahn”, which can also be found via GoEuro. The “Deutsche Bahn” also has some intercity bus routes called IC Bus.
busradar.com, an intercity bus search engine, does not only compare intercity buses, but – depending on filter settings – also train rides and carpooling (BlaBlaCar). The results can be sorted and filtered, and have additional information such as e.g. amenity points.
Anyone who wants to compensate the CO₂ emissions of his travel, this can be done on atmosfair for instance. More information about CO₂ emissions will be available in another entry soon.
Please note that most of the mentioned sites work best for traveling in Europe.