- Lubuntu (for "Light Ubuntu") is a variant of Ubuntu that is less demanding in it system requirements (Pentium II or Celeron CPU, 128MB RAM instead of a Pentium 4 @ 1GHz and 512MB RAM for Ubuntu).
- Puppy Linux is a small-footprint distribution that can be run from a live CD or a USB flash drive. They offer Slackware-based and Ubuntu-based distro as "latest releases" (for newer hardware) as well as well as a long-term-support distro called WaryPuppy (for older hardware). I've successfully loaded and run WaryPuppy on a 700MHz Celeron-based system with 128MB RAM and a 20GB hard drive, and it worked reasonably well.
Integrated Development Environment (IDE)