Mentorship, diversity, & equity
The Women in Psychology (WiP) group at Harvard, which I started as a grad student. Check out our "Resources" and "See the Stats" pages for information, and follow us on Twitter for announcements about our public events and workshops.
Writing & professional development
Free copy of Barbara Sarnecka's book (and associated resources), The Writing Workshop: Write More, Write Better, Be Happier in Academia. I highly recommend being a part of/forming your own local writing group as outlined in Sarnecka's book.