By the end of 19th century, the Cossack population of the Russian Empire was about 3.5 million. Right before the bolshevist revolution there were about 4.5 million Cossacks. They lived mostly along the southern borders of the Empire (see the map of the Cossack Hosts). During the Civil War and two World Wars Cossacks suffered heavy losses.
Now the Cossack population is estimated around 4-5 million people. It is more difficult to track it down, since many Cossack descendants don't live anymore in traditional Cossack areas. Some of the Cossacks found themselves outside of Russia after the split of USSR, many Cossacks emigrated with the White Army right after the Civil War, many Cossacks left Soviet Union during WWII. Nobody knows their numbers exactly.
The first formally registered Cossack organization since the destruction of the Cossackdom was founded under the Moscow Chapter of the VOOPIK, All-Union Society for Preservation of the Monuments of History and Culture( Vsesoyznoe Obshchestvo Ohrany Pamiatnikov Istorii I Kul'tury). It was officially registered in the very beginning of 1990. After that the Cossack cultural centers, societies, etc. began growing like mushrooms after the rain. All the years of the communist rule the Cossacks remembered who they are and were waiting for the first chance to re-establish their communities, no matter to what part of the country life brought them, even if it was not a traditional Cossack region. (For example, during Soviet Years many Cossacks were moving a lot around a country. Now there are Cossack communities in Moscow and St.Petersburg areas.)
At the beginning of 1990's, Cossacks began re-claiming their special status throughout the former USSR. The location of the Cossack communities pretty much corresponds to the traditional hosts that were there before the revolution (see the map of Cossack Hosts), but now there are significant groups of Cossack population in Moscow, St. Petersburg and other major cities. They organize themselves in unions and stanitsa's too.
Beginning from the early 90's one can see sometimes on the streets people in Cossack uniforms. The Cossack communities were among the first targets of the communists, under Lenin, Stalin, and all the following rulers. And yet, Cossacks have survived all the purges, and now again represent a significant force within Russia, and it is likely that their importance and influence are to grow.
There are Cossacks that found themselves within the borders of newly independent republics (e.g., Semirechen Cossacks). In many regions with strong Cossack influence, they managed to establish parallel self-government structures.
The Cossack movement now has three major dimensions:
Cossack movement doesn't look very monolithic these days. There are all kinds of unions and associations, Cossacks are having different ideas of what Russia should be like and how it should proceed with the economical and political development. But there is an increasing trend to stay away from any parties and non-Cossack movements. Cossacks view themselves as servants to Mother Russia, not some political parties.
Even though Cossacks have not settled their organizational structure yet, there is a strong sense of unity, and if there are any collisions of interests, Cossacks are likely to act as a single front, even though there are attempts to divide them again into Reds, Whites etc. There might be some misunderstanding and rivalry on the level of the organizational/administrative leaders (atamans), but on the level of communities Cossacks see each other as brothers no matter what union, host or region they belong to.
Today the Cossacks represent a group with high potential in many spheres of Russian politics, economy and military. In the regions where they have strong communities, Cossacks are capable of generating popular support of their demands (e.g., Northern Caucasus). The more Russians get disappointed with the Russian government, the more they look up to the Cossacks who are seen as defenders of interests of ethnic Russians both within and outside of the borders of the Russian Federation.
In some areas Cossacks already participate in the law enforcement, where they represent an element that is independent from Internal Ministry, which is known for corruption. The Cossacks act on the volunteer basis, they maintain order in their communities better than any police units. In the area of the Don host, for example, they patrol the streets, railway stations, etc. Even in Moscow and St. Petersburg there were examples when the Cossacks maintained order during mass events (like celebrations around New Years Eve, when there can be outbursts of street hooliganism, etc.).
There are already examples of Cossacks performing the border guard functions (Northern Caucasus, Southern Siberia, Far East). In perspective, there will be more Cossack military type colonies on the borders, where they will be performing military and border control functions while being farmers or hunters in the "time off", the way it used to be in the beginning of this century. With the mind-bugging length of Russian border, it is very difficult and costly to maintain regular type of military outposts everywhere. It seems that Cossacks will be taking over that function from the army and internal ministry at least in the areas of their traditional settlements.