Parted Magic (free) has been my go-to Linux boot disc since I first downloaded it several years ago. Not only does it boot quickly and provide easy access to the Gnome partition editor, GParted, it provides access to the Memtest86+ memory testing utility and numerous other handy apps on board.
The latest version of Parted Magic, 6.0, has ditched the pretty but occasionally problematic graphic boot menu for a more compatible character-based boot menu. I actually had boot issues on only one machine out of the maybe one hundred I used version 5.x on--and that one was running some rare hardware--but it was a bummer. I look forward to 100% successful boots with 6.0.
Memtest86+ has been my memory diagnostic tool of choice for over 10 years now. If your memory has an issue, this character-based (DOS-like) utility will find it. It's found under Extras in the boot menu along with the character-based MHDD which will gather and display your hard drive's S.M.A.R.T. info for you.
By default, Parted Magic boots (from CD, USB drive, or PXE) to a live version of Linux (including a low-ram version) and provides access to the aforementioned GParted partition creator and editor, a system profiler, several imaging utilities, a benchmark utility, ISO image editor, a couple of file managers, and a host of other useful stuff.
It's Linux stuff, but Linux understands the FAT, NTFS, HFS, and HFS+ and most other file systems these days…so you can use Parted Magic to copy files to and from your Windows or Mac hard drives. If you have an operating system go belly up, and need to get those vital data files off, one of the file managers on Parted Magic can be a godsend.
About the only other thing I can say about Parted Magic, besides that it's a must-download for your toolkit, is that it's free. That's always nice--however, a donation to the creator and maintainer wouldn't be out of order if you find it useful.