Loooonnnggg time fave tool.
You'll find it mentioned in my [link|http://twiki.iwethey.org/Main/DebianChrootInstall|Debian Chroot Install] instructions. I was first pointed at it by Heather Stern (part of [link|http://www.linuxgazett.net/|Linux Gazette]'s Answer Gang, back ~1998 or so. It's saved my ass several times (so blame Tom for me still being here).
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[link|http://www.lnx-bbc.org/|LNX-BBC] is more tools (including an X environment) on a business-card form-factor CD. Very useful in its place -- has more capabilities than TRB but ties up the (or an) CD drive. Nice bit is that it usually fits into a PDA case making it handy to tote. And of course [link|http://www.knoppix.org/|Knoppix] is there when you can carry a full-sized CDROM.
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There are scads of micro-Linuxes as well, including Trinux, mu-Linux, and others. [link|http://lwn.net/Distributions/index.php3#floppy|LWN has a good list of floppy distros] (CDs follow). All told, 358 listed distributions.