Communio, edited in the English language by David L. Schindler. Founded in the wake of the Second Vatican Council by Hans Urs von Balthasar and Josef Ratzinger with Henri de Lubac SJ and Louis Bouyer, with the support of Karol Wojtyla in Poland (later John Paul II), it has never sold in huge numbers but has had and is having a huge if indirect impact on the Catholic Church through the fact that many of its contributors and editors have been appointed bishops and cardinals, often placed in key positions (Francis George, Marc Ouellet, Christoph Schonborn, Angelo Scola, and of course Ratzinger himself are the most obvious). Communio theology is an expression of the Catholic ressourcement or "back to the sources" movement that partly influenced the liturgical movement and the Second Vatican Council, and is certainly now influencing their interpretation and consolidation under Pope Benedict.
All around the United States there are Communio circles that meet to discuss articles from recent issues, but in the UK there seem to be too few subscribers in any one place to make this viable. Even in the States, many readers find Communio hard going. (Second Spring was founded, in part, to offer a more accessible way into this tradition of Catholic thought.) But if you are seriously interested in creatively orthodox Catholic thought, Communio is indispensable. The journal has a News page which is a good place to start, and this has links to a number of articles.
I have selected several important Communio articles for our own site, which you can find under author in our Articles section linked from the menu at Second Spring. Look for example under Bouyer, Crawford, Granados, Hanby, Henrici, Kaveny, Lopez, Melina, Nault, Olsen, Ouellet, Schindler (D.L.), Schindler (D.C.), Schonborn, Scola, Sicari - as well as, of course, Popes Benedict and John Paul II. There are also several recent ones on Catholic social teaching to be found in the articles section of our "Economy" site - Abela, Berry, Cloutier, Healy, Schindler (both), and Walker. And for an introduction to Balthasar go here. I hope to write more about Communio and education in the future.