+1 for herbe! It’s a neat, minimalist tool. However, I’ve taken to using phillbush/xnotify instead. Configuration via .Xresources, supports rendering icons, etc. The author also has a bunch of other neat X tools.
The herbe also supports configuration using ~/.Xresources file - there is patch for that in the GitHub repository :)
herbe has just landed in the FreeBSD Ports Collection: https://cgit.freebsd.org/ports/commit/?id=1b6314a7773b47e18f5da903035bca74a235f35c
I use dunst which is also fairly minimal but in general I prefer to have as few as possible. Notifications for things like incoming e-mail just prevent me from getting work done so I don’t have them.