I have been a big fan of Fantasy Grounds
. It is not a free product, but it is a one time expense. There is a light version and a full version. I highly recommend that everyone buy the full version. The full version allows you to host a session and DM. The light version will only connect to a full version and allow you to be a player only. (No 'upgrading' from light to full, so just go full!) I have not regretted the purchase more than three years later. If a bunch of you go in together you can get a bundle discount. (See the store for more info.)
A D&D 4E ruleset has been developed by members of the FG community and is available for free. I've been playing in a campaign for a little over a month now using it. (They keep adding new features to the ruleset, so we keep moving to the newest version.) By the way, FG comes with a standard d20 ruleset. There are commercial rulesets available for purchase. (Such as Savage Worlds.)
One thing I always note when price comes up is that the cost of the software ensures a level of commitment from players that free programs cannot provide. Paying a one shot price (not a subscription) for a piece of software that I have used for over three years in a handful of campaigns was well worth it in my mind.
That being said, if you must have free, take a look at MapTools. The developers are clearly focused on a slightly different goal from FG, but it looks like a good product.
I do not keep up on which ones run on a mac, but this chart might be useful.
iConvention VTT Comparison Chart
Note there are two pages, so do not ignore the programs on page two. Some folks may have gotten FG running on a mac, but you should check the FG forums for that info. It has certainly been discussed.
As to timezones, using FG I have gamed with people in Asia, Australia, the Americas, and Europe. People all over the world have different schedules and sometimes it works out.
In any event, if you start using a virtual tabletop (vtt) I hope you have fun. Just make sure that you make a decision that works for you. (On that note, I should mention that there is a demo for FG so you can try before you buy. It is not full featured but you can get the basic idea and see if you can get it working. Post questions to the FG forum, they are helpful folks.)