Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Of the abundance of Linux distributions available, quite a few have sprung up whose stated goal is to give you the most Linux in the least amount of space. They run in low memory, require relatively little disk space to install, and at the same time are immediately useful and powerful: you can boot and get right to work in seconds, not minutes or hours.
I've rounded up five of what I feel to be the most notable and interesting of the tiny distributions. Each of these distros were tested in a relatively tight system configuration -- 128 Mbytes of RAM, and no more than 8 Gbytes of disk space -- so if they performed well in that environment, odds are they'd perform even better in a more sophisticated one. In addition to how well they worked as both a live distribution and as an installed operating system, I also looked at the mix of applications provided in the basic install and how useful they were in a general desktop setting.