There are bus connections from Riga to virtually any point in Europe, Russia, Ukraine and Belarus.
Riga bus station (LV, RU, EN, GER)
Bus itineraries: quick search (LV, EN, RU)
Ecolines coach service (EN, RU, LV, LT, PL, DE, BG, EE)
Eurolines coach service (LV, EN, RU)
Nordeka coach service (LV, EN, RU)
Bus Europe - booking service for 22 countries (LV, EN, RU)