
I ran a campaign for a good long while with RPTools, specifically MapTool.  It's free, works pretty well, has a lot of built in functions, and isn't too taxing on systems.  Setup can be a little frustrating but there are a metric tonne of tutorials on youtube and other avenues to help you out.  It's fantastic for D&D as they have modules for all the different versions.  Let me know if you have any questions about it and I'll help out if I can.  Best of luck to you!