Core

Stack Overflow uses a WISC ^[1] stack via BizSpark ^[2] (We Graduated!):

• Operating System Microsoft Windows Server 2008 R2 x64 ^[3]
• Web Server IIS 7.5 ^[4]
• Database SQL Server 2012 ^[5] running Microsoft Windows Server 2008 Enterprise Edition x64 ^[6]
• Language C# ^[7]

Software Development Tools

• IDE Visual Studio 2012
• Framework Microsoft ASP.NET ^[8] (version 4.0) on .NET 4.5
• Web Framework ASP.NET MVC 4 ^[9] with MiniProfiler ^[10]
• View Engine Razor 2 ^[11]
• Browser Framework jQuery 1.7.1 ^[12]
• Data Access Layer LINQ to SQL ^[13] and Dapper ^[14]
• Cache / Additional Data redis 2.4 via BookSleeve ^[15], with serialization via protobuf-net ^[16]
• Source Control Mercurial ^[17] (after a switch from Subversion)
• Source Control Integration Mercurial ^[18]
• Compare Tool Beyond Compare 3 ^[19]

External Bits

Code used in Stack Overflow that is not included as part of the development tools:

• reCAPTCHA ^[20]
• DotNetOpenId ^[21]
• WMD ^[22] - Now developed as open source in the project PageDown ^[23]
• Prettify ^[24]
• Google Analytics ^[25]
• Team City ^[26]
• Cruise Control .NET ^[27] (almost gone)
• HAProxy ^[28]
• Cacti ^[29]
• MarkdownSharp ^[30]
• Nginx ^[31]
• Kiln ^[32]
• Orion ^[33]

Miscellaneous

• WordPress on Linux ^[34] (for blog.stackexchange.com ^[35])
• WebSocket ^[36] (for real time updates ^[37]; custom C# implementation ^[38])
• Bandwidth used by Stack Exchange sites ^[39]

Content

• License Creative Commons Attribution-Share Alike 3.0 Generic ^[40]
• Standards OpenSearch ^[41], Atom ^[42]
• Host two datacenters: PEAK Internet ^[43] OR and Peer 1 ^[44] NYC

Hardware

• 11 Dell R610 IIS web servers (three dedicated to Stack Overflow, two for Meta and development):
  • 2x Intel Xeon Processor E5640 @ 2.66 GHz Quad Core with 8 threads
  • 16 GB RAM
  • Windows Server 2008 R2
  • Two drives
    • RAID 1
    • 2x Intel 320 300GB SSD (RAID 1)

• Three Dell R720xd database servers (two in New York City, one in Oregon, using SQL AlwaysOn Clustering):
  • 2x Intel Xeon Processor E5-2680 @ 2.70 GHz
  • 384 GB RAM
  • 15 drives
    • Mirrored Pair for OS
    • 12+1 Intel 710 SSD RAID10 for databases
  • SQL Server 2012

• Three Dell R720xd database servers (two in New York City, one in Oregon, using SQL AlwaysOn Clustering) (Stack Overflow dedicated):
  • 2x Intel Xeon Processor E5-2680 @ 2.70 GHz
  • 384 GB RAM
  • 15 drives
    • Mirrored Pair for OS
    • 12+1 Intel 710 SSD RAID10 for databases
  • SQL Server 2012

• Two Dell R620 HAProxy/nginx servers:
  • 2x Intel Xeon Processor E5-2650 @ 2.00 GHz
  • 32 GB RAM
  • CentOS 6.x

• 2 Dell R610 Redis servers:
  • 2x Intel Xeon Processor E5640 @ 2.66 GHz
  • 96 GB RAM
  • CentOS

• Two Dell R610 Service servers for tag engine/search:
  • 2x Intel Xeon Processor E5660 @ 2.80 GHz
  • 72 GB RAM

• One Dell R610 Linux backup server running NetBackup:
  • 1x Intel Xeon Processor E5640 @ 2.66 GHz
  • 32 GB RAM

• One Dell R610 Linux management server for log files:
  • 1x Intel Xeon Processor E5640 @ 2.66 GHz
  • 32 GB RAM

• One Dell R620 Orion monitoring server:
  • 2x Intel Xeon Processor E5-2650 @ 2.00 GHz
  • 32 GB RAM

• Two Dell R610 VMWare ESXi:
  • 1x Intel Xeon Processor E5640 @ 2.66 GHz
  • 16 GB RAM

• 2 Cisco 3945 routers
• 2 ASA 5520 firewalls
• 6 Cisco 2960S 48 port switches with FlexStack

Sources:

1. Stack Overflow's New York Data Center (Server Fault Blog) ^[45]
2. Designing for Scalability of Management and Fault Tolerance (Server Fault Blog) ^[46]
3. What Was Stack Overflow Built With? ^[47]
4. Stack Overflow Server Glamour Shots ^[48]
5. Technology and SEO profile for stackoverflow.com ^[49]
6. Stack Overflow and DVCS ^[50]
7. Stack Overflow Network Configuration ^[51]

[1] http://stackoverflow.com/questions/177901/what-does-wisc-stack-mean
[2] http://blog.stackoverflow.com/2009/03/stack-overflow-and-bizspark/
[3] http://www.microsoft.com/sqlserver/2008/en/us/r2.aspx
[4] http://www.iis.net/
[5] http://www.microsoft.com/sqlserver/en/us/default.aspx
[6] http://serverfault.com/questions/34846/in-place-upgrade-from-windows-server-standard-to-enterprise-or-datacenter
[7] http://en.wikipedia.org/wiki/C_Sharp_%28programming_language%29
[8] http://en.wikipedia.org/wiki/ASP.NET
[9] http://www.asp.net/mvc/mvc4
[10] http://nuget.org/packages/MiniProfiler
[11] http://weblogs.asp.net/scottgu/archive/2010/07/02/introducing-razor.aspx
[12] http://jquery.com/
[13] http://msdn.microsoft.com/en-us/library/bb386976.aspx
[14] http://code.google.com/p/dapper-dot-net/
[15] http://nuget.org/packages/BookSleeve
[16] http://nuget.org/packages/protobuf-net
[17] http://mercurial.selenic.com/
[18] http://mercurial.selenic.com/
[19] http://www.scootersoftware.com/
[20] http://recaptcha.net/
[21] http://www.dotnetopenauth.net/
[22] https://code.google.com/p/wmd/
[23] http://code.google.com/p/pagedown/
[24] http://google-code-prettify.googlecode.com/
[25] http://www.google.com/analytics/
[26] http://www.jetbrains.com/teamcity/
[27] http://confluence.public.thoughtworks.org/display/CCNET/Welcome+to+CruiseControl.NET
[28] http://haproxy.1wt.eu/
[29] http://www.cacti.net/
[30] http://code.google.com/p/markdownsharp/
[31] http://nginx.org/
[32] http://www.fogcreek.com/Kiln/
[33] http://www.solarwinds.com/products/network-management/network-monitoring.aspx
[34] http://blog.stackoverflow.com/2009/12/blog-outage-backup-policies/
[35] http://blog.stackexchange.com/
[36] http://www.websocket.org/
[37] http://meta.stackoverflow.com/questions/125677/new-feature-real-time-updates-to-questions-answers-and-inbox
[38] http://meta.stackoverflow.com/a/168148/152859
[39] http://meta.stackoverflow.com/a/172340/152859
[40] http://creativecommons.org/licenses/by-sa/3.0/
[41] http://www.opensearch.org/
[42] http://en.wikipedia.org/wiki/Atom_%28standard%29
[43] http://www.peakinternet.com/business/hosting/?stackoverflow
[44] http://blog.serverfault.com/2010/10/29/1432571770/
[45] http://blog.serverfault.com/2010/10/29/1432571770/
[46] http://blog.serverfault.com/2010/09/10/1097492931/
[47] http://blog.stackoverflow.com/2008/09/what-was-stack-overflow-built-with/
[48] http://blog.stackoverflow.com/2009/12/stack-overflow-rack-glamour-shots/
[49] http://blog.stackoverflow.com/2008/04/our-dedicated-server/
[50] http://blog.stackoverflow.com/2010/04/stack-overflow-and-dvcs/
[51] http://blog.stackoverflow.com/2010/01/stack-overflow-network-configuration/

Hmmm, the list seems incomplete. I don't see Unicornify ^[1] in there...

[1] http://unicornify.appspot.com/