...I use "bookmarks"...

What items have you ever used as a bookmark? What is the most unusual item you’ve ever used or seen used?

a prepaid cellphone card 

 folded Post-It note to be thrown away

a marker pen

small cards I like to collect 

I also collect tags

The above stuff are what I usually used as bookmarks. The marker pen is a last resort and only if for a very short period of time. I don't want to damage my books. I have some bookmarks though, but sometimes I misplaced them.

I noticed that some people fold a page on the book they're reading to mark what page they'd stop. As much as I possible, I will avoid this especially if my book is new.