I like
DM Genie. I mostly use it for character-building, but it has tools to roll attacks and automatically subtract HP from the person that was hit, for example. It can also automatically add buffs. It also has a nice following, so there are a lot of people that may have already added the splatbook/adventure/whathaveyou that you're using, so you don't have to make it from scratch.
The con of DM Genie is that it's not free, but I found it very worth what I paid for it.