My Sony PRS-650 marks books as read or unread on the screen.
But basically my TBR list is everything I own p- or e-. As some books I like to re-read, especially series, when a new books comes out, I will re-read the series to fit everything together.
I do keep a WTB (Want to Buy) list of books and authors I would like to pick up. Sometimes they are additional books by authors I already have (in some cases e- copies of pbooks I own). Others are new authors that look interesting. I keep this the old fashioned way, on paper.
I have several pieces of paper with jotted notes for books to get. But I also have a Note on my iPhone to jot down interesting books I see in book stores.
I have well over 100 titles on my WTB list.