The following information is taken from Everything2, from 'The Smell Of Rain' node:

If you've wondered why the ground, or the road smells a bit odd when it rains after a long dry spell, wonder no more... The smell is given off by Streptomyces bacteria, a genus belonging to the Actinomycetales order of Gram-positive eubacteria, also called actinomycetes.

The bacteria grow in damp, warm earth before fine weather dries out the soil, which then blows around as dust. During a dry spell, actinomycetes produce spores that are released on contact with moisture. Rain hitting the ground kicks up an aerosol of water and soil and you breathe in fine particles of soil containing the bacteria.

If you invite somebody to smell a plate of these bacteria grown in the lab, they always comment on how it smells just like the soil after rain.

Actinomycetes are also a source of many of our current antibiotics. Apparently, actinomycetes were also responsible for the distinctive smell of Glasgow's old underground system.

Source: The NewScientist: reader contributions by Duncan Simpson and Cary O'Donnell