Not that long ago (2016) they had:

  Servers:

  SQL Servers (Stack Overflow Cluster)
   2 Dell R720xd Servers
  SQL Servers (Stack Exchange “…and everything else” Cluster)
   2 Dell R730xd Servers, each with:
  Web Servers
   11 Dell R630 Servers
  Service Servers (Workers)
   2 Dell R630 Servers
   1 Dell R620 Server
  Elasticsearch Servers (Search)
   3 Dell R620 Servers
  HAProxy Servers (Load Balancers)
   2 Dell R620 Servers
  Redis Servers (Cache)
   2 Dell R630 Servers
  VM Servers (VMWare, Currently)
   2 Dell FX2s Blade Chassis, each with 2 of 4 blades populated
   4 Dell FC630 Blade Servers (2 per chassis)
   2 Equalogic SAN PS6000-series
  Machine Learning Servers (Providence)
   2 Dell R620 Servers
  Machine Learning Redis Servers (Still Providence)
   3 Dell R720xd Servers
  LogStash Servers
   6 Dell R720xd Servers
  HTTP Logging SQL Server
   1 Dell R730xd 
  Development SQL Server
   1 Dell R620 

  Network:

  2x Cisco Nexus 5596UP core switches (96 SFP+ ports each)
  10x Cisco Nexus 2232TM Fabric Extenders (2 per rack)
  2x Fortinet 800C Firewalls
  2x Cisco ASR-1001 Routers
  2x Cisco ASR-1001-x Routers
  6x Cisco 2960S-48TS-L Management network switches (1 Per Rack)

https://nickcraver.com/blog/2016/03/29/stack-overflow-the-ha...