The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
Calculog is a simple calculator with an extra feature: It logs the history in the background and lets you send it to your email address anytime. You can also edit the history to add text notes before sending. It is a simple app, but is thought to become the default calculator for every user.