Natural indicators have been mentioned elsewhere. Indicators like red cabbage juice and tumeric are visual indicators. The reference mentioned here is about a natural indicator that is based on olfactory rather than visual observation. In fact it is called olfactory titration [local] and it allows students to investigate the smell of onions and use the odor to find the end point of a titration between HCl and NaOH.

Aspirin, buffered aspirin and Enteric Coated Aspirin.