Halloween is even more fun when the kids' costumes are themed!
Growing up, we NEVER bought Halloween costumes. We improvised with clothing and items we could find around the house. Inexpensive? Yes. Memorable? No.
Not to say a homemade Halloween costume can't be memorable - there's a kid at my son's school who made his own costume and went as an iPhone last year. But we were never that creative. Even if I tried going the homemade route, I'd likely mess up, spend more money, etc.
Instead, we do the themed costumes with the boys as often as we can.
Yoda and Anakin:

Yoda got happier once he figured out what people were putting in his bucket.
Buzz and Woody:

My personal favorite of all time.
(The following year, and I can't find a picture of it to save my life, they went as Spock and Captain Kirk.)
Mario and Luigi:

A close second!
This year, they'll be Captain America and the Hulk. My little one, who is to be the Hulk, wanted to wear his Hulk muscle shirt, ripped up pants and his Hulk smashers. We bought the costume anyway.
Have you ever done a Halloween costume theme? What were your favorites?