List of fighting games featuring Akuma
Super Street Fighter II Turbo (1994) - secret character, his strongest incarnation. Because of this, he suffers from SNK Boss Syndrome.
X-Men: Children of the Atom (1994) - secret character
Street Fighter Alpha (1995) - secret character
Street Fighter Alpha 2 (1996) - normal character, Shin Akuma is a secret character
Street Fighter EX (1996) - secret character
X-Men vs. Street Fighter (1996) - secret character
Pocket Fighter (1997) - secret character
Marvel Super Heroes vs. Street Fighter (1997) - normal character, the final boss is called "Cyber Akuma", also a secret character (PlayStation version)
Street Fighter EX Plus (1997) - secret character
Cyberbots (1997) - Secret "Zero Akuma" mech, it's also worth noting that its name is a pun on Evangelion's Unit-00's Japanese name, Zerogouki. (Console versions only)
Street Fighter Alpha 3 (1998) - normal character, Shin Akuma is a secret character
Street Fighter III: 2nd Impact (1998) - both Akuma and Shin Akuma are secret characters
SNK vs. Capcom: Match of the Millennium (1999) - secret character
Street Fighter III: 3rd Strike (1999) - normal character
Capcom vs. SNK: Millennium Fight 2000 (2000) - secret character
Marvel vs. Capcom 2 (2000) - normal (but locked on consoles) character
Capcom vs. SNK Pro (2001) - normal character, Shin Akuma is a secret boss character
Capcom vs. SNK 2 (2001) - normal character, Shin Akuma is a secret character
SVC Chaos: SNK vs. Capcom (2003) - normal character, Shin Akuma is not a hidden charracter
Capcom Fighting Evolution (2004) - Shin Akuma only, secret character
"Akuma," "Shin Akuma," and "Zero Akuma" card
Namco x Capcom (2005) - boss character
List of non-fighting games featuring Akuma
Super Puzzle Fighter II Turbo (1996) - secret character
SNK vs. Capcom: Card Fighter's Clash (1999) - both "Akuma" and "Zero Akuma" cards
SNK vs. Capcom: Card Fighters 2 Expand Edition (2001) - "Akuma," "Shin Akuma," and "Zero Akuma" card
Namco x Capcom (2005) - boss character