I have missed pat distribution Pokemon events and I am really sad about that. The question speaks for itself, but remember for the current ones, I am asking for events around March 17 2015.
It is important to note that there have been hundreds if not thousands of events over the years. If we are going to include them all, from all countries as well, we would spend weeks on this question. Luckily there are already list of events you can find here:
http://www.serebii.net/events/2003.shtml if you want to find a specific event easily or when a certain Pokemon was distributed.
http://bulbapedia.bulbagarden.net/wiki/List_of_event_Pokémon_distributions For a more coherent list of events based on year.
Generally, almost all Pokemon have had a special distribution at some point, but the most well known are the Event exclusive distributions. There have also been events for games outside the main series in a vast number of ways, but they are also highlighted above. Events that are currently ongoing can be found here:
As for future events, there will most likely be more of them. Since they haven't happened yet I can't provide a source, but there is no reason there shouldn't come any more.