By messing around i found out that a few Apple II games which were later ported (or not) to other systems were made in BASIC (and can be extracted from their respective disks), the gfx & music aren't exactly stellar quality of course, but the games engines remain essentially the same and could be used to port /improve
these games, make editors or anything else.
3D Docking Mission.
Accounts Receivable (Tool).
Battle For Normandy.
Bronze Dragon Side II.
Cartels & Cutthroats.
Dungeon Campaign - Wilderness Campaign.
La Cite Perdue (fr).
Planetary Construction Set.
Sands Of Mars.
Shard Of Spring.
Star Warrior - Starquest.
The Adventure - Only the fittest shall survive.
The Infiltrator (Asm source).
The Standing Stones.
Tigers In The Snow.
Ultima I (California Pacific Computer version).
War in Russia.
Also some Amstrad CPC games were also made in BASIC (in French unfortunately):
L'Affaire Vera Cruz (Available in English).
Harry & Harry - La boite de Rajmahal.
Harry & Harry - Mission Torpedo.
La cité perdue.
La secte noire.
Les Griffes De La Nuit.
Les Templiers d'Orven.
Meurtres En Serie (ported on ST).
Meurtre A Grande Vitesse.
Une Affaire En Or.
Warrior & Warrior +.
I didn't investigate more but there's surely more of them on these machines or on other 8 bit (MSX, Thomson, etc.).
Download "CiderPress" from http://a2ciderpress.com/
in order to extract the .dsk files from the Apple 2
Download "ManageDsk v0.20h" from http://cpcrulez.fr/emulateurs_UTIL-DSK-manageDSK.htm
in order to extract the .dsk files from the Amstrad CPC.
EDIT: Added "Meurtre A Grande Vitesse".
EDIT: fixed download link for ManageDsk.