Coloured paper edges
I saw these in Waterstones the other day, and they really caught my eye. Coloured pages edges - matching the full colour block on the cover. The coloured edges turning the book into these blocks of solid colour are very eye catching and effective.
I know you've got to do whatever you can to sell a book these days - so it's pretty much any gimmick goes... but I like these. Shame the cover treatment and the type are nothing to write home about.
(Excuse the shitty pictures)

Famous people with vinyl
This did the rounds a good bit ago... but it's good fun and there are some amazing pictures here.