I have a Commodore 128, but this is a platform-neutral question.
I have a lot of experience with a lot of operating systems (including DOS, Linux, BSD and AmigaOS) but I never used CP/M much back in the day.
Since I have compatible hardware, what are some good resources (books, web sites, etc.) to help me get started? What software (ideally freely distributable) would be useful to start doing things (e.g. terminals, games, text editors, etc.)?