Here is a very informative site, which collects information on the horizontal (or lateral) gene transfer:
http://www.panspermia.org/virus.htm
Here are a couple of links to posts by Jerry A. Coyne:
http://whyevolutionistrue.wordpress.com/2011/04/13/how-important-is-lateral-gene-transfer/
http://whyevolutionistrue.wordpress.com/2010/05/01/aphids-nab-pigment-genes-from-fungus/