The Ultimate Community Map Pack that contains volumes 1 through 4.
We've also got a surprise for those who have been UCMP-deprived. If you haven't tried any of our mappacks, we've put together a massive compilation of all of them. 44 maps in on file weighing in at 567 MBs. Broadband-only need apply, it goes without saying.
If you put any of the maps on servers, please send us the IP's and we'll put them up on the website so people can find them. Either email them to email@example.com or post it on our forums: ucmpforum.org.