The Mac Mini is just a gorgeous device. It is beautiful, small, silent, powerfull yet energy efficient. When idle, it uses around 20 watts. I'm using one of the first Intel-based Minis with an Intel Core Duo chip, running at 1.6 Ghz.
I want to use this mini as an expensive router and download host. I could have used something embedded, such as one of those router boxes that costs about 70 euros, but no, I want to do some more with my router, such as downloading, etc. It is the only device in my house that is allowed to run 24/7 so it has to be a bit more powerful if I want more than just routing. I know that this mini was like 600 euros or something back in the days, and that is quite some money to spend on something that is now only a router. However, when I was still running Mac OS X on it, I didn't do much more with it than I will now, it will actually do more.
I am assuming that you want to run Linux exclusively on the Mac and that Mac OS X will be wiped off.
To get this puppy running Debian Linux (Lenny), you need to first boot the Mac with the (Snow) Leopard OS X boot CD and startup the diskutilily.
You need to create at least two partitions: one for the root file system and one for swap. The most important step is to select 'options' under the partition layout screen, and select Master Boot Record partitioning instead of the other 2 options. Do NOT use GUID or Apple Partition Map.
Now, boot your regular Debian Linux boot CD, I use the regular network installation CD. When you get to the partitioning screen, do NOT auto- partition the hard disk. Just reconfigure the existing partitions you just made using Diskutility. So the large partition will be configured as "/" and made bootable. The small partition must be configured as swap.
After the installation finishes, just install GRUB in the MBR and reboot. If all went alright, you will see a non-blinking folder on a gray background for a couple of seconds, after which Linux will boot. If you get a blinking gray folder with a question mark, something went wrong.
It seems that if configured properly, after the EFI boot mechanism fails to find a system folder on some Mac partition, the legacy BIOS emulation seems to kick in, and star to search for something to boot.
The Mini has only one network card, so another one is necessary to run it as a router. I bought some no brand USB2 to 100 MBIT NIC (Bus 005 Device 003: ID 9710:7830 MosChip Semiconductor MCS7830 Ethernet) which seems to run smoothly.
card with a dual gigabit card.
I guess you will need to mod the Mini but it will allow true gigabit speeds on all interfaces.