A friend of mine sent me these two main calendars for trail/ultra races:
- Ontario Ultra Series/Ontario Trail Series - This is geared toward the Ultra crowd (>42.2km) but there are other shorter distances at most of the events.
- 5 Peaks Ontario - A series of five trail races, typically featuring "Sport" (around 5K) and "Enduro" (around 12K) options.
There are other one-off events here and there, I've mostly found out about them on Running forums. The Running Mania site has a great Ultra/Trail section, most upcoming races are discussed there.
If anyone has any other sources of information, please let me know!