what type of internal memory do most digital cameras use? (not talking about SD cards or other removable storage)
NAND/NOR flash is the best guess.  Not much info pertaining to design specs are available for digital cameras, so the best way to answer which make/model it houses is to visually inspect it by disassembling it down to the mainboard. 
Check for a chip marked with the logo of one of the flash manufacturers that existed in 2006 as a reference point  (Spansion, Intel, ST Microelectronics, Samsung, Toshiba, Sharp, Micron, Renesas, Macronix, Silicon Storage Technology)  Find the 8-20 character model number on the chip and Google it - you should be able to find design specs for the chip that confirm it's NAND/NOR.

Trim the leads to separate the chip from the mainboard and proceed to destroy the chip by way of introducing several physical cracks.  Use reasonable protection such as sticking it in between 2 layers of duct tape, wear gloves &amp; goggles then smash it.

No publicly available software solution seems to exist as expected since everyone sets their own standard.

**Smashing the entire device and/or incinerating it - however valid - is in a grey area legally and ethically