Personally, I reformat my Windows 7 machine regularly. As a developer, I tend to install beta products, and just try out almost any application. This means that occasionally, I need to start fresh. This requires some discipline (I store no data at all on my C: drive--there's a lot written about how to avoid this, but in general, I partition my physical drive into multiple logical drives and store all data on drive D:). In this scenario, I need some simple way to install all my standard applications on the new installation of Windows.
But as a normal person, you may occasionally purchase a new computer, and need to install all your standard applications on the new computer. I don't know about you, but at least 80% of the applications I run come from the Web. I use Picasa, NotePad++, EverNote, DropBox, Adobe Reader, and lots more applications that I need to install from the Web.
If I need to install these one by one, it takes way too much time.
Instead, I use a great service called Allmyapps (http://allmyapps.com) . This service currently only works only on Windows PC’s but it really helps. Once you set up an account you can tell it which applications you are most likely to use on your computer. When you need to re-install them, you instruct AllMyApps to install your entire set. It downloads the most current version of each application, and installs each application for you. It's a tremendous time saver.
There are similar services that may work better for you. Ninite.com is popular, and provides a similar service for Windows users. On the Mac, I'm not aware of any similar services, but the Mac App Store and the third-party app Bodega track apps you have purchased or installed for you (but in each case, you must install your applications manually). You may also find the popular MacUpdate service helpful.
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