I have a spare machine with an 80GB HDD and 512 MB RAM. I want to try a Linux distro that's very small (and free too). What should I try?
For a computer with those specs you really don't need to worry about size. You can run Linux on far smaller machines just fine.
A simple option would be Ubuntu - it does most things right out of the box, so while not quite as lean as some other Linuxes, it is an easy one to try out if you have never used one before. If you want, you can even run it off a CD to try before you install.
There is even an Ubuntu Stackexchange so you can get assistance easily if required.
But with those machine specs, you don't need such a light-weight distribution. You can install any, like Debian or Ubuntu... you can check many of them to see which one fits better in the idea you have for that machine at Distrowatch.
Update: Just discovered Tiny Core Linux, and seems also a very interesting option for a very small Server/Desktop.
Update 2, 2013 Review:
Another 2 options I've found looking for
old computers on Distrowach:
Distributions described as lightweight
[BasicLinux]- A very lightweight distribution capable of running on an Intel 386 and 3 MB of RAM.
[Damn Small Linux] - "light enough to power a 486DX with 16MB of Ram"
[Lubuntu]- light weight in comparison to Ubuntu.
[Puppy Linux] - light weight relative to most other Linux distributions.
[SliTaz] - 25 megabyte distribution
[Tiny Core Linux] - 10 megabyte distribution
[Xubuntu] - light weight in comparison to Ubuntu and Kubuntu.
It's powerful and yet light.
Use this download link.
I realize this is an old question, but I just had to add my 2 cents worth:
1) I would highly recommend Bodhi Linux. It has a fantastic user interface (nice eye candy) yet requires only these minimum hardware specs: 300+MHz CPU, 128MB RAM, and 2.5GB hard drive space (see their about page)
2) As for a free distribution, you don't have far to look, they are just about all free ;-)
Also, Bodhi is an Ubuntu derivative, meaning it is not only easy to install, but you can benefit from all the packages in the Ubuntu repository.
What I recommend you, is Debian. It's stable and runs well on most architechture.
I have myself Debian on my iBook G4 (30Gb HDD, 256 MB RAM), what could use most of your memory, is the desktop manager. If you only browse on the internet, check emails, chat with some friends, you could install GNOME. If you need more memory, I recommend you Fluxbox, it's a lightweight desktop manager, but it's not easy to use (not always user-friendly).
I've tried Ubuntu, but it's not as lightweight and stable as Debian...But it's more beautiful :)
It's your personal choice, it depends on what you need.
I have a Ubuntu virtual machine on a 4GB USB memory stick with enough space for a copy of VMware player, so I can have a full Linux/X/Eclipse development machine any time I need one.
To create it, I just downloaded a standard Ubuntu VM 'appliance' with most of the right components pre-installed, added a few tweaks, and off I went.
Lubuntu 12.04 works for me, but others have problems installing.
Build your own distro http://www.linuxfromscratch.org/lfs/