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Show [SQL*Plus]

show all
show system-variable
show errors
show errors [function|procedure|package|package body|trigger|view|type|type body|dimension|java class name-of-function-procedure-etc]
show lno
show parameters param-name
show pno
show recyclebin
show release
show repfooter
show repheader
show sga
show spool
show sqlcode
show title
show user

show parameter

show parameter init-parameter
show parameter is a convinient way to show the value(s) of the requested initialization parameter(s).
Behind the scences, show parameter XYZ does something like select name, type, value from v$parameter where name like lower('XYZ').

show sga

Displays the total size of the sga as well as the sga's fixed size, variable size, db buffer size and redo buffer size.
Here's a sample output of show sga:
SQL> show sga

Total System Global Area   61771936 bytes       
Fixed Size                    73888 bytes
Variable Size              44249088 bytes
Database Buffers           16384000 bytes
Redo Buffers                1064960 bytes
61771936 = Total System Global Area = 73888 + 44249088 + 16384000 + 1064960.

show error

Getting the errors of the last compiled function/procedure/package/type:
show errors
Getting the errors for function foo:
show error function foo

show recyclebin

This command is new in Oracle 10g and shows the content of the recycle bin.
show recyclebin

show user

Shows the currently logged on user.

show describe

Shows the current settings for the output of describe.