Welcome to the FX codes
Blue codes are old, they can be found everywhere or in a few places.
Red codes I've found in mags and sites but I can't get them to work. (they are probably fake)
Green codes- I don't have this game so I can't test it.
Orange codes are discovered by me, you might find them elsewhere but they were here first.
Sega Dreamcast
Sonic Adventure
Black, Silver and Gold Chao's - Black is in the Egg Carrier jail, Silver, in the Mystic Ruins you need to push in a rock near the waterfall and it drops from the waterfall and Gold, you need to replace the gold egg in the store with the rock.
(if you like these chao things then search these places often because they do reappear if your lucky)
Open Twinkle Circuit Early - Beat the game in this order of characters Sonic, E-102, Big then Amy, now play as Knuckles or Tails, and the Twinkle Circuit Pass will be waiting for you.
Picture Disc - I'm shocked I haven't seen this anywhere, I found this on 9/9/99. Simply turn on the Dreamcast with the lid open, go to the Music screen, put the Sonic Adventure disc in and close     Cool Huh!
(This also works on several other Dreamcast games)
Tails flies with open mouth - I'm surprised I haven't seen this either. Beat the game with Tails, now DO NOT SHUT THE POWER OFF, play as Tails again and he will fly with his mouth open (he does in character select too)
(Shutting the power off ends this code from working, to get it to work again you have to do it again)
Tails flies with open mouth 2: Face of fear - Enter a stage and get hit, now Tails will still fly with his mouth open, but it looks like he's scared
(Shutting the power off ends this code from working, to get it to work again you have to do it again)
Breaking Into Another Characters' Level - Too much for me to write but you can find them in other sites (below are tricks I found in the broken levels)
Tails in Speed Highway: Goin' Down - Press A to fly then hold X (who said sonic was "the fastest thing alive")
Knuckles in Twinkle Park - When you get to Pleasure Castle get over the fence at the pool and hit that check point, now die, when the level reloads parts of it will be clear (if you hit a spring the game freezes)