Reinstaller works by connection to http://www.windowsphone.com, asking you to log-in, and then scanning the website for all the apps you ever installed on your phone. This generates a list within the app of all the apps you've download and installed to your Windows Phone device(s). Reinstaller will then let you choose which apps you would like to reinstall. Once you've done this you can start the re-installation process.
App re-installation is facilitated via deep links to the Marketplace app (i.e. the app will prompt you to install via the Marketplace app). This does still have to be done app by app, due to limitations in the Windows Phone platform APIs, but it's a relatively painless process and a lot easier than using the Windows Phone site directly, or trying to remember what you had previously installed.
The new version of Reinstaller has a re-designed user interface, making it easier to use. But, the most welcome changes are the improvements in the way the generated app lists are handled. It's now possible exclude trial apps from the generated list and apps can be browsed by Marketplace category (e.g. puzzle+trivia games). The first of these is particularly useful because trial apps are most likely to be among those you do not want to reinstall (having tried, but not purchased the app or game).
Other updates include the ability to run the app under the lock screen and bug fixes for app count and duplicate entries issues.
Reinstaller can be downloaded from the Windows Phone Marketplace for free.