If you just want to use it to learn and not actually use full time, I suggest either slackware or build your own. At first, you will have no idea what you're doing, but after you do a lot of reading and expiermentation, you will know much more than if you had used a "mainstream" distro like red hat or mandrake. The problem with the "mainstream" distros is that they are geared to getting it up and running as soon as possible. They have fancy wizards, aliases, look, feel, and other niches that make it easy for windows users to migrate to linux. But the problem with this is that you don't learn as much and you get familiar with the distro's commands (ie. typing "cd.." in many mainstream distros will move one directory up. but in another system w/o this alias, it will not work). If you learn the actual linux commands and what not, you will be able to use ALL the linux distros. Remember, don't quit because you get stuck on what seems like a limitation of the software. The great thing about open source is that the only limitations of the software is whatever limitations you set yourself. Have fun.