The largest project I have worked on was as lead infrastructure architect for the Domainz .nz TLD project.
This involved replacing a number of manual systems that maintained everything to do with .nz with a fully automated and highly available solution for Domainz to run their business and everything on the Internet ending with .nz to get resolved.
The manual system only allowed for domain updates 1 time per day, 5 days a week. The new system could run 7 *24 with updates every 1/2 hour (although twice per day was the business determined schedule).
The solution was based on SQL 6.5 (2 way replication) and IIS (NT4) with BSD & Bind for the name servers. When Domainz was sold and lost ownership of the registry, the TLD solution was replace with a 100% open source solution which cost many times more than the original solution and only provided the registry function (none of the finance/billing that was also required by Domainz).