The following are known Action Replay Codes for Pokemon Emerald Version on Nintendo Game Boy Advance (GBA).
These codes were also known as GameShark Codes and in most cases will work on both devices.

These codes are for the U.S. version of the game.

Master Code – Must Be On
d8bae4d9
4864dce5
a86cdba5
19ba49b3
a57e2ede
a5aff3e4
1c7b3231
b494738c

Infinite Money
c051ccf6
975e8da1

No Random Battles
b505db41
6e39ea4e

Battle: 1 Hit Kills
b63a2aeb
e4849958
c4ee7b57
20069537

Battle: Press R = More EXP
43d8ac45
0d3b349a
3dc869e3
d39c09b2

Have All Pokeballs
085938bb
99ff313d
2dafd739
5d796510

Have All TMs/HMs
c6511ec5
0f15c8e0
4689920d
5cff6ffe
8631b929
014933df
10257b84
3365249c
aea23f29
64edd481
1c91630e
c663e711

Pokedex is Complete
93cc476a
8c7b6c01
03650473
df38618e
67c11314
8556956f
bb87c867
94118b68
f6be2d41
c51b2ef3
fa701ea5
7d896f37
c9adf3d5
20700d38
974ddbcc
406cf0ac

Set Low Game Time
49daa9d9
0ebf4a2a