Here is a list I've reposted a few times:
DoPo said:
I've heard people have luck using Skype + Google Docs for communication and map/sheets respectively.
But as for actual software:
MapTools - I've heard it's the best
iTableTop - one of the most popular
OpenRPG - another of the most popular virtual table software
VirtualDaivve - I don't really know much about it
I don't have any experience in them but I've seen them mentioned in topics such as this one.
Hope it helps.
EDIT: ah yes, I forgot about the one the poster below suggested - Gametabe is another one of the most famous programs.
I've personally not used any but MapTools is still the best I've heard of. As in, being recommended the most. I haven't had a look at roll20 but it tends to crop up more and more.
Sorry, I can't be of more help but for what it's worth, I doubt the software matters that much - since people are happy to work with just cobbled together Skype/other chat and Google Docs/alternative, it's probably more up to the group than the software.