As of 2021-12-16 (PostgreSQL 12.6)
address_standardizer
address_standardizer_data_us
amcheck
aws_commons
aws_l
ambda
aws_s3
bloom
btree_gin
btree_gist
citext
cube
dblink
dict_int
dict_xsyn
earthdistance
fuzzystrmatch
hll
hstore
hstore_plperl
intagg
intarray
ip4r
isn
jsonb_plperl
log_fdw
ltree
oracle_fdw
orafce
pageinspect
pgaudit
pgcrypto
pglogical
pgrouting
pgrowlocks
pgstattuple
pgtap
pg_bigm
pg_buffercache
pg_cron
pg_freespacemap
pg_hint_plan
pg_partman
pg_prewarm
pg_proctab
pg_repack
pg_similarity
pg_stat_statements
pg_transport
pg_trgm
pg_visibility
plcoffee
plls
plperl
plpgsql
plprofiler
pltcl
plv8
postgis
postgis_tiger_geocoder
postgis_raster
postgis_topology
postgres_fdw
prefix
rdkit
sslinfo
tablefunc
test_parser
tsm_system_rows
tsm_system_time
unaccent
uuid-ossp
Update 2018-10-18 (I'm pleased to see they added hll)
address_standardizer
address_standardizer_data_us
amcheck
bloom
btree_gin
btree_gist
chkpass
citext
cube
dblink
dict_int
dict_xsyn
earthdistance
fuzzystrmatch
hll
hstore
hstore_plperl
intagg
intarray
ip4r
isn
log_fdw
ltree
orafce
pageinspect
pg_buffercache
pg_freespacemap
pg_hint_plan
pg_prewarm
pg_repack
pg_similarity
pg_stat_statements
pg_trgm
pg_visibility
pgaudit
pgcrypto
pglogical
pgrouting
pgrowlocks
pgstattuple
plcoffee
plls
plperl
plpgsql
pltcl
plv8
postgis
postgis_tiger_geocoder
postgis_topology
postgres_fdw
prefix
sslinfo
tablefunc
test_parser
tsearch2
tsm_system_rows
tsm_system_time
unaccent
uuid-ossp
As of 2014-09-17 (RDS PostgreSQL 9.3.3).
btree_gin
btree_gist
chkpass
citext
cube
dblink
dict_int
dict_xsyn
earthdistance
fuzzystrmatch
hstore
intagg
intarray
isn
ltree
pgcrypto
pgrowlocks
pg_trgm
plperl
plpgsql
pltcl
postgis
postgis_tiger_geocoder
postgis_topology
sslinfo
tablefunc
tsearch2
unaccent
uuid-ossp
btree_gin
btree_gist
chkpass
citext
cube
dblink
dict_int
dict_xsyn
earthdistance
fuzzystrmatch
hstore
intagg
intarray
isn
ltree
pgcrypto
pgrowlocks
pg_trgm
plperl
plpgsql
pltcl
postgis
postgis_tiger_geocoder
postgis_topology
sslinfo
tablefunc
tsearch2
unaccent
uuid-ossp
