In DS9 episode 1x07 "Q-Less", Q appears inside the Deep Space 9 station, where he interacts with various characters and is seen in Quark's Bar by the guests, especially during a fistfight with Benjamin Sisko.
Besides being Deep Space 9 a station known to be frequented by members of many different species that can potentially have seen him or directly ...
The Q appear to be immune to changes in the timeline and can change the timeline at will and are present (singularly) in all timelines and universes.
In All Good Things, Q alters the timeline of Earth by creating (or rather prompting Captain Picard to create) an inverse time rupture which explodes backwards in time to destroy the human race. Q and Picard, ...
From the Apocrypha (i.e. non-canon) section of the Memory Alpha entry on The Traveler:
According to the Q Continuum series, The Traveler even has a significant reputation among the Q Continuum; when the female Q learned that Wesley had been chosen by The Traveler, she was almost impressed by this fact.