New major features:

Enabled autocomplete for \ commands (including files for \i, DSNs for \c, fields for \pset) and context-based completion for SELECT, UPDATE, INSERT, DELETE queries and variables
Added a \copy command to copy data between different databases (local files are not supported yet)
Added a \crosstab command
Added a \G exec command that outputs in a vertical format (ala mysql)
Added a \watch command
PAGER support
Added \o and \qecho commands to send output to files or pipe to other system commands
Added the optional FILE argument to \g and \gx to send their output to a file or pipe to other system commands; the commands also support passing formatting options
Support the unaligned, tuples_only, html and asciidoc output formats
Context propagation and cancellation (CTRL+C) for in-flight queries

New Database Support:

Other fixes/improvements:

Added a -G for vertical output (see \G above)
Added a -q flag and associated QUIET environment variable to enable quiet output
Implement the auto option for the \x command
Fixed issues with escaped and other special characters when \set-ting variables
The \begin transaction command now supports all of Go's transaction isolation levels, and read-only support
\d updated to print relations, foreign keys, and other constraints for tables
The \d* commands now provide more information and work better with some databases; they also do a better job of hiding system objects by default
\d command support for the Snowflake database
Now sets the ROW_COUNT variable for DML queries
Fixes table titles

Notes:

CONTRIBUTING.md has been updated with details on implementing \d* / autocomplete for other databases