The three Baltic countries are all part of the Schengen agreement, so there are no border checks when driving between Estonia and Latvia. There’s usually no problem taking hire cars across the border but you'll need to let the rental company know at the time of hire if you intend to do so; some companies will charge an additional fee.
The following bus companies all have services between Estonia and the other Baltic states:
Ecolines (www.ecolines.net) Major routes: Tallinn–Pärnu–Rīga (seven daily), six of which continue on to Vilnius; Tallinn–St Petersburg (six daily); Tartu–Valga–Rīga (two daily); Vilnius–Rīga–Tartu–Narva–St Petersburg (four daily).
Lux Express & Simple Express (www.luxexpress.eu) Major routes: Tallinn–Pärnu–Rīga (up to 13 daily), eight of which continue on to Panevėžys and Vilnius; Tallinn–Rakvere–Sillamäe–Narva–St Petersburg (four daily); Tallinn–Tartu–Võru–Moscow (daily); Rīga–Valmiera–Tartu–Sillamäe–Narva–St Petersburg (nine to 10 daily).
Eurolines (www.eurolines.lt) Two daily Tallinn–Pärnu–Rīga–Panevėžys–Vilnius–Kaunas–Warsaw buses.
Valga is the terminus for both the Estonian and Latvian rail systems, but the train services don’t connect up. From Valga, Estonian trains operated by Elron (www.elron.ee) head to Elva, Tartu and Tallinn, while Latvian trains operated by Pasažieru vilciens (www.pv.lv) head to Valmiera, Cēsis, Sigulda and Rīga. There are also direct trains to Tallinn from St Petersburg and Moscow.