My Blu Spy from Team Fortress 2 was actually not too hard. The suit wasn't too hard to find, and a lot of the props were easy to find/make too. The hardest part was probably the ski mask and getting the shape of the mouth and eye holes right.
When first worn I had about 16 paper masks on hand, 14 being of my friends, one of 'Awesome Face' and one of 'GENTLEMEN'. (Seen here: http://media.giantbomb.com/uploads/0/7120/941549-gentlemen_super.jpg)
Now I've got masks for all of the classes as well.
Third Place Male Costume (Halloween Dance '09)
Second Place Costume (RKsocal Teens Halloween Party '09)