The APRS Mapping web site has to be one of the most useful tools available to VHF DX'ers. Since it relies on real time 2 Meter APRS data it can help to determine both tropo and e-skip openings. You can visit it at:
My only complaint with the service is that it does not automatically update or refresh itself, the way that other tools such as the 144mhz Propagation Logger or the VHF QSO Real Time Maps do. For users of the Firefox web browser that isn't a problem. You can simply add an extension called "Reload Every" and set the interval that you want the page to refresh itself. I have mine set for about 10 minutes, and that seems to work well.
I haven't checked, but I'm sure that you can find a similar tool for Internet Explorer. Hope this helps others.