All sites that are not actual Q/A sites are not listed.
- SE Chat
- SE Data Explorer - Often referred to as SEDE
- SE Dev Test site
- SE OpenID
- A few redirector sites (some of which changed names/domains in the past)
- Ask Ubuntu (You can refer to AU with
[ubuntu.se]as a quick shorthand link in comments) - MathOverflow
- Nothing to Install (later Web Applications)
- Mi Yodeya
- Basically Money (later Personal Finance & Money)
- Fantasy (later Science Fiction & Fantasy)
- Health (later Medical Sciences)
- Ask Ubuntu (You can refer to AU with
- Web Socket, the socket that powers the realtime page
Strictly speaking, Area 51 is not in the list, but it's easily known and accessible (it's in the top-left dropdown menu of all sites, though).
Other (sub)domains of Stack Overflow:
- SO Chat
- Stack Overflow Blog
- SO Blog Alternative, redirects to above
- Stack Overflow "Facebook" tag - Redirects to SO with the facebook tag (previously, a truckload of facebook-related tags)
- Stack Overflow Jobs - Discontinued as of April, 2022 (previously redirected to SO Talent)
- Stack Overflow Careers - Discontinued as of April, 2022 (previously redirected to SO Talent)
- Stack Overflow Talent - ↑↑↑
- RADS - Used to be a dedicated redirection site. Closed now (redirects to SO)
- SO Developer Console - Not a test site! Only used by staffs
- DevDays - This has been discontinued.
- SO Email Host
- SO Email Host Alt
Other apex domains that the Stack Exchange company owns:
StackAuth (The server that enables cross-domain login so you don't have to login separately to SO, SU, SF, AU, etc.)
Stack Overflow for Teams - since the removal of Teams from the main site domain and sidebar