This is your chance to meet agents, editors, publishers, peers, and (perhaps surprisingly) even fans! The first time someone I didn't know brought a book to me to sign and said they enjoyed my work (heart be stilled!) was at a convention.
Besides the in-person networking, you may be able to pitch your projects to decision-makers, and attend (or participate) in panel discussions, workshops, and other seminars in order to hone your craft. Besides the business opportunities, Con's are also just a great time to have fun with like-minded people over the course of one-to-four days.
Here are some of the better, or larger conventions, or those that particularly interest me, my interest being: horror, genre fiction, science fiction, fantasy, and associated sub-genres; these are listed in absolutely no order at all:
www.thrillerfest.com (Thrillerfest) - New York City, yearly
www.bookexpoamerica.com (Book Expo America) - New York City, yearly
www.worldhorrorconvention.com (World Horror Convention) - Rotates cities in North America, yearly
www.worldfantasy.org (World Fantasy) - Rotates cities in North America, yearly
www.fact.org/dillo (ArmadilloCon) - Austin, TX, yearly
www.campnecon.com (Necon) - Rhode Island, yearly
www.anthocon.com (Anthocon) - New Hampshire, yearlywww.killercon.com (Killercon) - Las Vegas, NV, yearly
http://events.latimes.com/festivalofbooks (L.A. Festival of Books) - Los Angeles, yearly
www.norwescon.org (Pacific Northwest Sci-Fi & Fantasy Convention) - Seattle, WA, yearly
www.britishfantasysociety.co.uk (FantasyCon) - England, yearly
www.stokerweekend2011.org (Stoker Weekend) - Rotates cities in North America, yearly
www.readercon.com (Readercon) - Massachusetts, yearly
www.midsouthcon.org (Mid Southcon) - Tennessee, yearly
http://bizarrocon.wordpress.com (Bizarro Con) - Portland, OR, yearly
http://www.horrorfindweekend.com/about.html (HorrorFind Weekend) - Gettysburg, PA, yearly
http://www.mocon.indianahorror.org (Horror in Indiana) - Indianapolis, IN - yearly
http://home.earthlink.net/~basfa/t39main.html (TusCON) - Tuscon, AZ - yearly
http://milehicon.org/ (Mile High Con) - Denver, CO - yearly
In addition, here is a link that lists another SIXTY conventions related to horror, fantasy, science fiction, film, etc. around the country.
Need more reasons to attend a Con? Read some great advice by Susan Denney at Writing-World: http://www.writing-world.com/basics/conference.shtml
Hope this helps, and keep writing!