Linux.org lists numerous catogries of Linux distributions. If your computer hardware is decidely antique you may want to look over the "Minimalist" choices. Most are designed to act as firewall/gateway servers, but some such as DemoLinux may be a suitable choice if you just wanna see what its all about without actually devoting hard drive space to it.

If you've already decided to actually install a full blown operating, and have more comtemporary hardware, then you still have several choices. Here's what Linux.org mentions about some of the more popular flavors of Linux.

Red Hat Linux is one of the most popular distributions in the world. It is geared toward all levels of users. The beginner will find ease of installation and configuration. The advanced user will find a robust and highly configurable computing enviroment adaptable to any need.

Slackware Linux

Slackware Linux is compatible with most Intel PC hardware. Slackware will provide stellar performance on high-end systems, including support for symmetric multi-processing (up to 16 processors), PCI, and special code optimizations for the 486, Pentium, and Pentium Pro and AMD Athlon

SuSE Linux

The current version is SuSE Linux 8.0. It contains 7 CD-ROMs (or 1 DVD) with more than 1,500 applications. SuSE can be installed and maintained easily with their YAST program, currently in version 2. SuSE Linux is also available for IA 32, IA 64, PowerPC, Alpha, and S/390.