Maintenance was written by Fruitfulcode to add an offline holding page to WordPress. There are many of these plugins around and they all do the job suggested, this is the one I choose for my sites at the moment. It’s slick and I’ve never experienced any trouble with this at all. It’s customisable too.
WP Maintenance Mode was created by Designmodo to do the same thing. I’ve not yet tried this plugin personally but do like the look of the different styles it seems to have. – I am sure the same is achievable with all these plugins, so I can only comment on what I have tried personally.
Coming Soon Page was created by Bob Riley with the slight difference of adding a email field to a holding page. Cool concept. Look forward to trying this one on upcoming sites.
How many maintenance plugins would we need?