Warning: reports in the comments and on the internet state that the current version installs a lot of unrelated software/malware, such as OpenCandy, AVG Search Toolbar and Amazon search. It's also reported to set Amazon as your default page.
I use the BSD licensed http://CrystalMark.info/software/CrystalDiskInfo
It is written in MFC, but the code is otherwise very clean.
It can send email alerts.
It can be configured to start at logon unless it detects the system running on batteries.
It has a fancy taskbar display (enabled from the "Function | Resident" menu entry).
It is localized to 31 languages.