Xubuntu 12.04 is the only lighter weight distribution that is getting Long Term Support (LTS) from Canonical. Support for Xubuntu LTS will be for 3 years compared to the life-cycle of 18 months and shorter than the 5 years given to Ubuntu 12.04 LTS.
Xubuntu uses the XFCE 4.8 desktop which is less resource hungry than Unity or KDE and comes in two flavors, 32 bit and 64 bit. It is also an installable Live distribution and is based on Linux kernel 3.2 series. Read more