I’ve got a new calendar system working here at Comixpedia.com. Please submit conventions, online events and local meetups to it. It’s very straightforward – when logged in, click on create content (upper right-hand side, under your username) and then choose EVENT.
Fill in all of the relevent detail in the box (especially the website URL if available) and then add in the start and end date/times for the event.
The right-hand Upcoming Events box shows the events coming up. Supposedly there is also a link to add this to your iCalendar but sans Mac I can’t test that function. Someone let me know if it works or not!