Currently, there is only one lithium cell available which is a direct replacement for AA alkaline cells. That is the one made by Energizer, given the designation L91.
There are other AA-sized cells available, but they are 3.6V and are meant for long-life, low-drain applications, like running the onboard clock of various electronic devices such as that found on your computer. Some people have found that they can just about drive 1 to 2 5mm LEDs directly using this cell, but I wouldn't really bother.
The other common lithium cells available are based on a 3V chemistry. These are the ones you normally put in to your cameras. They aren't really AA-sized. They are actually based on fractional sizes of the now defunct A-sized cells, so they are actually a little fatter than AA.
Lithium cells are lighter than alkaline, NiCd, NiMH and lead-acid cells of equivalent volume. They should be, since lithium is lighter than all the other metal bases used in the other chemistries. And lithium is a highly reactive metal, so cells based on this chemistry tend to have more energy than cells of equivalent volumes based on other chemistries.