The Megalithic Portal - Marvellous site listing many of the megalithic sites throughout the world, with very accurate information on getting to them. Warning: not ALL megalithic sites are listed on here, there does not appear to be a definitive list. So if you search (e.g.) for dolmen in a particular country and nothing pops up, there may still be dolmen there but no one has added them to the site. Still a damn good and very useful site though!
Dolmen Tour - this is the organised tour of dolmen in the Russian Caucasus / Gelendzhik region that I went on and is highly recommended.
Dr Trifonov's Pages on Piezoelectric Dolmen in Gelendzhik - Yes, I am not the only person to come up with this idea!
Dolmens of the Caucasus Russia Deciphered : Evidence and Megalithic Sites - Andis Kaulins. Dolmens of the Caucasus as stars of the heavens.