The NSLU2 is sort of a NAS (network addressed storage) where you have to use your own drives. Great for backups and whatever, but it can do so much more. There is alternative firmware such as Openwrt that can be installed on it to allow it to act as a router, You can find other firmware that will even allow the unit to act as a PBX. Those are specially distributions. We wanted to go all the way and install a full Linux distribution on it. Then the sky is the limit of what you can do with it. all of this only takes a few watts compared to the traditional server that uses hundreds of watts. True energy saving.
The NSLU2 has been named the "slug" for it's slow speed. Despite that it can do so many wonderful things. Another shortcoming is that you will not be able to use a monitor out of the box to watch it. You will have to use another computer or terminal to access it. Ironically I had an old text based unix terminal to work with it. That terminal has gone to a better place. Have actually converted the NSLU2 to Linux before but returned it to the original firmware when it looked like it might fizzle out. Fortunately the Arm processors (CPU) are now all the rage. I thought that I might see a zillion of the units on Ebay. Nope there were none. So that is a testament to how useful the unit is.
Note: if you have any drives connected or that have been connected to your NSLU2, make sure you can access them from another computer before you even start thinking about doing this project. If not you could lose valuable data!!!! Try this project at your own risk, as I will not be responsible for any or all issues. Get a linux professional if you feel uncomfortable with this project.