The Solovetsky Islands were the site of the first Soviet concentration camps, the SLON (Solovetsky special purpose camp), which held mostly political prisoners. The camp functioned from 1923 to 1939. The test was such a resounding success that Stalin eventually spread such slave-labor camps across the Soviet Union.
One of the best documentaries of the glasnost period, Marina Goldovskaya's "Solovetsky Power," reveals the history of the camp. Many prisoners were sent there for trivial reasons. For example, the literary scholar Dmitry Likhachev was arrested and sent to Solovki for an article on the relative merits of some letters in the old Russian alphabet.
Sibelan Forrester has some photos of what Solovki looks like today.
Prisoners arriving at Solovki in the 1920s