Make primary
============
ARFLAGS - flags for ar
CC - the default C compiler
CFLAGS - flags for cc
FC - the default FORTRAN complier
FFLAGS - flags for FORTRAN
GET - the program to use for SCCS get
GFLAGS - flags for get
LDFLAGS - flags for the linker
LEX - the default lexer
LFLAGS - flags for the lexer
MAKEFLAGS - default args for make
MAKESHELL - shell for make
PROJECTDIR - directory to search for SCCS files not in current directory
YACC - the default yacc implementation
YFLAGS - flags for yacc

Environment
===========
BROWSER - colon-separated list of browsers to try using for URLs
CDPATH - directories to use as a base for cd
COLUMNS - width for terminal screen/window
DATEMSK - template file for getdate()
DEAD - where mailx saves partial messages; defaults to ~/dead.letter
DISPLAY - default display for X11 programs
EDITOR - default text editor
ENV - a script to execute when starting sh interactively
EXINIT - commands to execute when starting ex
FCEDIT - the default value for sh's -e option
HISTFILE - file for terminal history
HISTSIZE - number of lines in the history file
HOME - home directory
IFS - characters used for field splitting, * expansion, and the read utility
LD_LIBRARY_PATH - directores to search for shared objects
LIBPATH - alternative for LD_LIBRARY_PATH
LINENO - line number in script before executing a command
LINES - height of terminal screen/window
LISTER - the command mailx uses when the folders command is given
LOGNAME - the user's login name
LPDEST - printer to use for lp
MAIL - the path of the user's mailbox
MAILCHECK - how often mail should be checked by sh
MAILPATH - where something mail-related is kept
MAILRC - mail configuration file
MANPATH - where man pages are
MBOX - where mailx saves read messages
MORE - default options for more (string of command line arguments)
MSGVERB - message components for fmtmsg()
OLDPWD - working directory before the last cd operation
OPTARG - used by getopts for argument storage
OPTERR - used by some getopts implementations for error stuff
OPTIND - used by getopts as the index of the next argument to parse
PAGER - the default output filter/paginator; defaults to more
PATH - where to look for executables
PBS_DPREFIX - default prefix for directives for qsub
PPID - parent process ID for the current shell
PRINTER - secondary printer to use for lp
PS1 - interactive prompt for shell
PS2 - continuation prompt for shell (i.e. for lines broken by \)
PS3 - prompt used for select loops in scripts
PS4 - prompt for script executed in debug mode
PWD - current working directory
RANDOM - generates a random number (from /dev/{u,}random?)
SECONDS - amount of time current shell has been around?
SHELL - preferrred command interpreter
TMPDIR - location for temporary files
TERM - terminal type for output
TERMCAP - tells system to use termcap rather than terminfo
TERMINFO - tells system to use terminfo rather than termcap
TZ - timezone
USER - alias for LOGNAME?
VISUAL - default visual text editor (subset of EDITOR)

Localization
============
LANG - language
LC_ALL - locale to use
LC_COLLATE - locate for collating order
LC_CTYPE - locale for character handling functions
LC_MESSAGES - language for messages
LC_MONETARY - locale for monetary values
LC_NUMERIC - locale for numbers
LC_TIME - locale for date/time formatting
NLSPATH - message catalogs

Unknown (not in standard)
=======
These are only briefly referenced as things that compliant applications
should avoid conflicting with. I don't know what these do.
-------
CHARSET
HISTORY
MAILER
NPROC
PROCLANG