Hi there, I am going through a similar thing myself at the moment so I thought I'd post.
I'm on a permanent contract, but our whole company has reshuffled so much lately that I've ended up temporarily surplus to requirements until they form some sort of new department. It means I literally have NOTHING to do all day - it's been going on since September and will go on til at least April.
As a seasoned 'time on my hands'er I may be in the (unfortunate) position to offer you some advice!:
*An obvious one, but plug any gaps you see or anything you see that can be done better or more efficiently, even if it's not part of your real role. I've ended up doing web development having no prior experience, just because the opportunity came up.
*Do as much training as you can, while you can
*Web surfing (mumsnet is a good one), but if that's not an option due to the arrangement of your office then you're stuffed on that one
*Language learning (via iPod) - I'm learning French 
*improve skills at desk, since you're office-based. For example could you learn how to build databases in Microsoft Access, or learn Excel VBA programming? It'll look like you're working even though you're not (chances are most of your colleagues won't care what you're doing anyway...) and you'll be gaining skills for your next contract
*can you work at home? I'm lucky as I can
*reading/sewing/knitting/puzzles - again may be difficult to get away with this depending on your office
*if you're pg (which I assume you are as you're posting here) just relax in the knowledge that it's not forever and you'll soon have plenty to fill your time!
*fall asleep at your desk and blame pregnancy and killer commute
*go to the toilet a lot and play with your phone!!
It's very sad to have to go to these lengths I know, especially as it's not our fault that we're in this position, but what can we do about it really if they're wanting to keep us on and not giving us any work to do? 