There are a lot of Linux ditributions which focus on the two big generic markets, server and desktop. Trying them all out can require significant effort, partitioning, choosing options, installing, configuring and testing. In the end, you might find that there really are more similarities than differences. Well you can forget about cleaning out partition space and waiting for the install to finish with these. Each one is boot and go, and addresses a specific niche rather than a broad generic space. You just might find yourself saying, "I didn't know Linux did that!"
Tom's Root Boot proclaims itself to be "The most GNU/Linux on 1 floppy disk." You won't find many people trying to dispute that claim, either. If you need a quick way to probe a system, this might be for you. It is strictly command line based, but is full of swiss army knife like power. Tinker with partitions, backup files over the network, recover a password file, wipe a drive clean, these are just a few of the things you might do. Tom's is especially useful with older hardware that might not be able to handle booting from CD or have enough memory to run a GUI system like Knoppix.
For the Media Artist
dyne:bolic provides a wealth of multimedia tools in a live CD form. It features audio and video processing tools and preconfigured internet streaming services. Boot straight into hosting your own internet radio show. [More]
Natural Language Processing
Morphix-NLP isn't for everyone. If concepts like grammar parsing and linguistic anaysis make your eyes roll, just ignore this, but if you have a passion for language, this could be for you. Here is an informative review.
Security Tools Distribution
Need high power, easily portable security tools? Knoppix-STD might be the ticket. This bootable CD is full of security tools and the documentation that goes with them. Scan for vulnerabilites and root kits, perfom forensic analysis on a compromised system, lure the bad guys into a honeypot, and much, much more.
LinuxDefender is Knoppix variant from BitDefender focused on virus incident recovery. Features like a SMTP antivirus/antispam proxy and virus scanning and disenfection make it a handy tool, but the built in NTFS write support using the Captive project makes it almost irresitable.
Want to Roll Your Own ?
Can you imagine a specialty distro of your own? Morphix is a modular distro building set that can put you on the path to creating a custom Live CD.