Good Morning,

there are some options depending on your needs. If easy-of-use is important you can have a look on:

yEd - Free but not Open source

http://www.yworks.com/en/products_yed_download.html#ReleaseNotes

Instead, a very powerful but a bit more difficult tool is:

Inkscape - Free and Open source

http://inkscape.org/

Finally, you can also try Libreoffice Draw and Dia but, in my opinion, they are not as good as the previous:

https://www.libreoffice.org/discover/draw/

http://dia-installer.de/download/linux.html.en