Impersonal Passives

Impersonal passives are a bit different from the normal ones but they are very interresting and they are very used in English.
http://www.aprenderinglesfacil.es/2010/05/exercises-voz-pasiva-impersonal-v.html

The Passive Voice

Here you've got a few links to study more passives.

http://www.englisch-hilfen.de/en/exercises_list/passiv.htm

http://www.ego4u.com/en/cram-up/grammar/passive/exercises?present-progressive

http://www.saberingles.com.ar/curso/lesson24/06.html