I've said this in another answer but I have two more suggestions:
1) No-one seems to have mentioned the great tool which my uni recommends. Its a 620K download called ER-Assistant
. Its basic plain black/white ERD
's but some like that. Its free and uses Crows foot notation, but windows only.
http://highered.mcgraw-hill.com/sites/0072942207/student_view0/e_r_assistant.html
2) Also Lucid Charts is decent. Its free to use until you reach very complex diagrams. Its web based so obviously platform agnostic
https://www.lucidchart.com