BusyCal is what iCal should have been. It continues to impress me the longer I use it, and recently I found a way to customize it that I did not know about previously.
Event details, like the one shown in the image here, can be customized from a much larger list than what BusyCal shows you by default. To access the Info Panel Preferences, click the small white "i" at the top right of the Event Info panel. (See item #1 in the image here. Note that you can only access these when the panel is floating on its own, not from the BusyCal sidebar.)
There are 18 different fields that you can use, and most of them are self explanatory (you can see an image in the gallery below), but I want to draw your attention to a few that I find very helpful. The first is "my URL" which gives you an easy way to link an event to a person from your address book. Simply drag the name from the address book to the my URL field and a link will be created (see #2 in image). Clicking that link will open their contact information in the Address Book.app.
You can also drag a person (or business) to the Location, Attendees, My Notes, or Notes field. For the latter two, you get more than just a link to their name, you get address and phone number information (see item #3 in image).
While you could use "Attendees" for the same purpose (and that field is shown by default), when you do that, BusyCal wants to know if you want to "invite" them or send them changes whenever you edit the event. That isn't a feature I ever use, so I didn't want to use Attendees, but my URL is almost perfect.