Im surprised no-one has mentioned AMOS/AMOS Pro. While people tend to have a bit of a love/hate relationship with it, it sold very well back in the day, and it's not uncommon to read on random computer development blogs,etc. about how a person cut their teeth on AMOS. In addition to this there's plenty of BASIC dialects since that have been inpsired by AMOS (Dark Basic/Dark Basic Pro being very similar to AMOS and also having had good commercial success itself).
Blitz Basic parallels this too, but not on quite the same scale.
Dpaint is a bit unusual in my opinion in that while it was hugely successful, and also gets mentioned in interviews with random developers fondly, never seemed to directly influence many big name graphics tools of today. There's never realy been something akin to what a dpaint9 or 10 (or whatever) might have been, but rather evolved in a different (less elegant for my tastes) direction.
