Scrivener and downloaded the free trial. (NB: I got the Windows version but it's available for Mac too.)
So far, so good!
I am loving the functionality of it, being able to create index cards and drag the entire scene to wherever I want it to go. There is a choice of views, including pin board and outline view, and you can keep all of your research and media in the same place rather than have to keep accessing other programs, all without having to leave your active document, thanks to the screen splits.
I am getting good use out of the 'full-screen' mode already, putting one more barrier between me and the internet where I already waste too much time, giving my willpower a bit of a boost. Then there's the 'compile' feature, where I choose the format (novel manuscript, PDF, MS Word, preview etc) and the software takes care of it all for me.
I foresee a long term relationship forming with me and my new writing buddy. Thankfully it's not all that expensive either, so I won't have to fork out too much when my free trial comes to an end.
I'm not getting anything out of this by the way, just in case you were wondering if I was on commission or something. By sharing my experience of it, I'd love to think it may be useful to somebody else too.
ETA: If you go ahead and download the free trial, there is an interactive tutorial that takes an hour or so to work through but was invaluable. I also sync my 'projects' with Dropbox, just in case I decide to pour tea or diet coke all over my laptop and it works out great, backing up automatically too.
Over to you... do you use a writing software or do you stick to trusty MS Word? Have you given writing software a go but had a negative experience and given up with it? Would you ever consider using writing software?
Updated 20/3/12: I bought my full license today, I really do love this program. Not convinced yet? I stumbled across these great tutorials too, covering outlining and brainstorming in Scrivener (Source: www.embowman.com/tag/scrivener/ )