Blinkbox currently have a 'sign-up, deposit £1, and we'll give you £5 free credit' here. The average film is £2.49 to rent (they vary from 99p to £3.49) so you should be able to watch 2 films for free and still have £1 for a cheap film.
They also have 'Movie Mondays' every week when you can rent one of 12 films for 99p, the 12 films change each week. I just caught it by chance on google, it doesn't seem to be advertised on the main Blinkbox site but the voucher codes are here You then have 30 days to watch the film, just like normal. Always worth a look to see if there's something you fancy watching.