I've been asked for unique venues around Seattle for conferences, hack fests, user group meetings, and the like.  Here's venues and types to check out. Feel free to add others. Startup standards & smaller:

• Capitol Hill Arts Center (CHAC)
• Columbia City Theater
• Coffee shops, especially large ones (Zeitgeist, Top Pot) and those that close early
• Pubs (less than 75, some less than 25): F.X. McRory's, Collins Pub, Tap House
• Market Theater at Pike Place Market

Larger:

• Seattle Asian Art Musuem (SAAM).  A gorgeous venue and and the novelty of the venue is a draw.  Killer place for a post-event party.  Some parking.
• Seattle Center "Northwest Rooms."  This is the land that the Space Needle is on.  I've seen events over 250.
• Large theaters: Egyptian Theater (has movie screen), Moore Theater, Paramount Theater, King Cat Theater (mostly concert venues)
• Town Hall Seattle
• Union Station "The Great Hall." The now-renovated giant Seattle train station in Pioneer Square.
• Community Center at Mercer View on Mercer Island.  Seats hundreds, and surprisingly decent bus service.  No atmosphere and nothing nearby.
• Other museums: Henry Art Gallery, Museum of History and Industry (MOHAI), Burke Musuem