We don't really celebrate Halloween here in Australia, but people still do have Halloween themed parties, dress up parties in general etc.
Went to my friend's sister's 21st a few months ago, I dressed as Ash Ketchum from the Pokémon anime and my mate here is the Red Ranger from the original Power Rangers. I got my costume from eBay (jacket, gloves and hat) so I just threw a black t-shirt under the jacket, wore light blue jeans, running shoes and usually wore a backpack. I also bought a cheap Pikachu doll and attached it to the right shoulder of the jacket using safety pins.
This was a dress up for my mate's birthday party earlier this year. The theme was "Like a Sir". Bought the top hat, gloves, bow tie and glasses online really cheap and the suit was just bought from a really cheap clothing store.
Dressed up as Mario while my same mate (the Red Ranger) dressed up as Luigi. Despite lacking a moustache in this photo, I certainly wore a fake 'stache when I went out to the party. My mate and I got a lot of attention that night, everyone wanted photos with us.
Went to a party at the university campus dressed as The Punisher. I had to hire a black coat, I bought gloves and a hip holster, bought a black t-shirt and black cargo pants. With the shirt, I made my own skull logo on it (since I couldn't locally buy a cool looking Punisher shirt). I made a stencil with cardboard, then painted the stencil onto the shirt with white fabric paint. It actually looked really cool. The gun belonged to my friend, it used to be a video game peripheral for Sega, I think.
Uh... this was a few years ago (2010) when I was down south for the Australian University Games of Sport. Every night was a themed party, and this night was "Classic sports stars" or something... so I bought a ridiculous wig, douche Kanye West glasses and wore my judo gi/jacket and the medals I won at that competition and arrived... as THIS.
For an actual Halloween party. This costume sucked. It was SO last minute... I hired it out the day before the party. Don't be lazy with costumes.
My mate and I dressed up as "Army of Two" except... my costume was retarded. It looked terrible and the vest was way too small. I did have a cool gun that lit up disco-style. My disco gun! We also decided to throw our "N7" stickers on our vests for a silly Mass Effect 3 reference... which only ONE person got the reference.
Last but not least... I dressed up as Darth Vader for a friend's birthday party. It's not a "proper" Vader suit... but it's close enough.