Clarion South Workshop - Brisbane, Australia (Clarion South - which runs every two years in Brisbane, Australia next one being in 2012 - is the most intensive professional development program for speculative fiction writers in the southern hemisphere)
San DIego Comic Con is far away the premiere comic con and media venue, taking place late July of each year in San Diego, California and noted for huge numbers and likely to be sold out. Also check out their facebook presence.
SMOFcon (this is quintessentially the convention to go to if you are interested in running a convention. There are others local events such as ConComCon and Construction for the San Francisco Bay Area that help local convention runners, but SMOFcon is THE event during the year of this type)
Star Wars Celebration is the official LucasFilms celebration of all things Star Wars. Like Worldcon and Westercon it is a movable feast, happening late spring to summer and locations as far afield as Japan, Colorado and Orlando, Florida. Check out the web site for the current location and dating.
WisCon - Madison, Wisconsin - Feminist
Science Fiction and Fantasy convention, annually over
Memorial Day Weekend in Madison, WI. Programming ranges
from fannish panels to readings, to becoming a writer.
Tiptree auctions benefit the annual Tiptree award for
DragonCon - Atlanta, Georgia This is one of the larger in terms of numbers for a science fiction convention with some emphasis on media and gaming and more.
Spring - Corflu -
This is a convention a gathering of the fanzine fan
community that migrates around the country.