You can download the Old Worlds here.
Server resets usually do not happen until there is a change in world generation algorithms. Which should happen very rarely.
Your work on the server should never be reset, so map resets are rarely performed unless circumstances are exigent. The 2012 world needed reset due to it becoming too crowded being a custom map. The 2013 World was only up for a month due to it being a failure, majority vote wanted a new world.
Worlds have not been reset since.
If you would like to play these on single player, place the world saves in the following directories:
Windows Vista/7 - C:Users<username>AppDataRoaming.minecraftsaves
Mac OS X - /Users/<username>/Library/Application Support/minecraft/saves/