Tried-and-true online chess playing software: Best bet for online chess play against others?

I'm an intermediate-level chess player new to the online gaming world. I want to be able to play chess online against other human beings. I *also* want to be able to play chess against my own computer. What software package(s) do you most highly recommend for being the "gold standard" for this sort of use? I'm looking for freeware as well as shareware. Thanks in advance!