I really only started with Discworld Conventions - Nullus Anxietas I, II, III and the UK Discworld Con (2010), but recently started attending the pop expos here in Australia - probably because they came to Adelaide for the first time last year.
Armageddon Expo 2011 in Adelaide, I saw Pat Tallman (Babylon 5), John Rhys Davies (Indiana Jones, Lord of the Rings), Christopher Heyerdahl and Ryan Robbins (Sanctuary), David Hewlett (Stargate Atlantis), Lance Henriksen (Aliens, etc) Robert Rankin (the author), and a few others.
Armageddon Expo 2011 in Melbourne, I saw Nick Brendon (Buffy the Vampire Slayer), Amanda Tapping (Stargate SG-1, Atlantis, Sanctuary), Robin Dunne (Sanctuary), Callum Blue (Dead Like Me), Marc Shephard (pretty much every show ever), Jeff Lewis & Sandeep Paraikh (The Guild) and a few more.
Oz Comic Con 2012 in Adelaide, I saw Ben Browder and Gigi Edgley (Farscape), Jewel Staite (Firefly, Stargate Atlantis), and Sean Astin (Lord of the Rings, The Colour of Magic).
I'm also planning to go to Armageddon in Melbourne in October, mainly because there'll be four Babylon 5 guests - Bruce Boxleitner, Mira Furlan, Pat Tallman and show writer/creator J. Michael Straczynski himself.
But before then, I've got a convention of my own to run - the Unseen University Convivium: http://ausdwcon.org