Where can I find the status of the released versions of Ubuntu?

Supported Releases

The currently supported releases will always be in the Wiki page of Ubuntu at:

https://wiki.ubuntu.com/ReleaseTeam#Releases.Current

This includes the current point release, Long Term Support release and regular releases.

Future (in development) Releases

For future (beta/alpha) versions of Ubuntu you may check:

https://wiki.ubuntu.com/ReleaseTeam#Releases.Future

Older (EOL) Releases

To know the supported dates of older (EOL) releases, you may visit:

https://wiki.ubuntu.com/ReleaseTeam#Releases.End_of_Life

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All releases can be downloaded at http://releases.ubuntu.com, for older releases you may look at http://old-releases.ubuntu.com.

The release schedule of any version will be at https://wiki.ubuntu.com/<codename>/ReleaseSchedule, replacing <codename> with the codename for that release, e.g. for Trusty Tahr:

https://wiki.ubuntu.com/TrustyTahr/ReleaseSchedule

Over at Wikipedia they keep this useful graph: