Not only one of the largest anonymous FTP collections on the internet, but also many people's first experience with the internet. In it's heyday, it comprised 10% of all internet traffic. It is still one of the official major repositories of RFCs, and mirrors many other smaller FTP sites.
wuarchive's current hardware (which hasn't been upgraded in a long while) consists of a Sun UltraSparc 2 Enterprise Server with 2 200MHz cpu's, 512 MB main memory, 180 GB of disk (RAID 5), and a 100Mb/s FDDI connection (with 100Mb/s ethernet as a backup). For comparison, to mirror all of TUCOWS only requires 110 GB and a recommended 64 MB memory.
wu-ftpd, originally called wuarchive-ftpd, was, yes, specially written to support wuarchive. While wuarchive still runs wu-ftpd, wu-ftpd is no longer maintained at Washington University in St. Louis.
wuarchive's decline started when the maintainer left, and nobody remembered to reassign wuarchive to somebody else. It went down for a while (nobody noticed right away), and was brought back up reluctantly. Currently there is a student-led push to turn management of wuarchive over to the CS department (and therefore the CS students).