In addition to the 9.04 release of the Ubuntu family, there are some recent and interesting lightweight distribution releases.
Easy Peasy 1.1 is a distribution for the wildly popular netbooks. Easy Peasy is based on Ubuntu which certainly is not known as the most lightweight distribution available. Even Easy Peasy defaults to software like OpenOffice.Org 3.0 and Firefox instead of lighter alternatives.
SliTaz 2.0 has been released, as well. SliTaz is a live system installable to a CD or a USB memory. It packs incredible amount of functionality in an image of 30 MB. See the SliTaz Handbook for more information.