Third-party replacement of the missing Windows 8 Start menu (I'm sorry, the Modern/Metro UI is not a start menu) has become quite the booming cottage industry. Stardock's Start8, a £3 alternative, has been the most seamless replacement for several months, but it's now going to have to share the larger part of that honor with Iobit's Start Menu 8, which is free. See all software and utilities reviews.
Start Menu 8 installs easily, mimics the Windows 7 Start menu to a tee—including the search field—and is just as configurable as that which it seeks to duplicate. You can show or hide the items such as administrative tools, control panel, documents, network, photos, etc., as well as disable the hot corners and the Windows 8 sidebar. You can also change the appearance of the start button to match Windows 8, XP, Windows 7, etc., though the menu itself retains the look of Windows 8. Here it loses out to Start8 which can morph its entire appearance and even simulate the Modern UI (formerly known as Metro) interface. Take a look at ArsClip review.
The Start Menu 8 installation routine will ask you if you want to install Iobit's Advanced SystemCare 6 software (the default is yes), but there's no advertising aside from that.
I found one minor bug. Start Menu 8 didn't interact with the taskbar properly when the latter was set to Auto-hide. Unlike Start8 and the normal Windows Start menu, which keep the taskbar displayed when it's open, Start Menu 8 would let it disappear, leaving itself hanging in mid-air, so to speak. This is a mere cosmetic issue which should be fixed soon.
To make a long review short: Small bug aside, Start Menu 8 works just like the Start Menu you wanted to see when you first got your Windows 8 PC. Do yourself and anyone else you know who's been saddled with a new interface they didn't really want a favor and install it or the slighty more capable Stardock app.