I'd like to make a high recommendation for Lubuntu for use on old (or simply mature) machines,
I've spent a couple of weeks trying various new releases of Linux distros, including various lightweight editions, and have tried Lubuntu for the first time - wow, it works brilliantly on my really old (1999?) Toshiba SP 4600 700Mhz 256MB laptop - my take-anywhere diagnostic machine. It has a good windows desktop manager and apps, packages (from Ubuntu) aplenty and runs my regular apps (Firefox, Sylpheed and Wine/Routerstats) at around 10% CPU and 60% RAM. I'm so impressed by it that it has now replaced Puppy Linux as the regular in-action O/S, although Wary Puppy is still there as an alternative boot for if I ever need it. All very surprising because I don't get on well with Ubuntu/Kubuntu/Xubuntu - all of which I find too restrictive re system settings.
I used the 'alternate' - slightly smaller - ISO download rather than the regular one, as it is suitable for machines with less than 384MB, and it installed cleanly and problem-free, although the process did take an hour or so. Updates came thru nicely and the Ubuntu packages I tried (Firefox and Wine) installed with no problems. The regular CD version also worked really well on my newest machine. which is 4 years old. The network install 24MB ISO version also worked, but was very slow on my 1.5M line - around 2+hours......I didn't repeat that!
On my other machines, I've now upgraded my main desktop from Mint 11 to Mint 12, and settled for PClinuxos 2012 on my 6-year-old Presario V4000 - so no change there, just upgraded to latest.
Distros tried included Slitaz, Slacko Puppy, DSL, Knoppix, Slackware, Arch, Ubuntu/Xubuntu/Kubuntu, Lubuntu, PClinuxosGnome//KDE/XFCE/LDE, Linux Mint, SUSE12, Debian, Centos (and probably a few more I've forgotten), some just on a desktop (large distros like SUSE etc), and some on the older machines.