I think that the Solidarity movement played a key role in the collapse of the Soviet Union, because it not only influenced the biggest country in the Eastern Block, (Poland which shared a long border with the USSR and previously served as a corridor for attack on the Soviet Union), it also managed to attain over 10 million members and supporters nationwide. Since the Solidarity movement or Solidarity itself was a trade union, it had great economic and social powers, which it used to bring people together, organize strikes, and protests, which were aimed at promoting certain ideals, or laws that the communist government banned or rejected. The movement was very successful at this, which was partially due to its large membership (10 million) and effective leadership under the former electrician Lech Walesa. Towards the final days of the Soviet Union, during the late 1980's, 99 percent of the seats in the Polish Parliament were held by Solidarity, this was clearly a blow to the Polish communists and proved that the Soviet Union was losing control of it's Eastern European neighbors. It also proved that the USSR was no longer willing to use any force, to enforce the Brezhnev doctrine (which meant that many other countries could freely change their governments and impose capitalist values without the threat of a Soviet-led invasion).
However, we can argue that the Solidarity movement was only one of the lesser factors that contributed to the downfall of Soviet power in Eastern Europe. The reforms introduced by Gorbacev, were far more effective in the long term, as they paved the way for new freedoms and ideals, which eventually led to the disintegration of both the USSR and the Warsaw Pact