InSync: a very nice option, that works with Linux, but also with Widows and Macintosh InSync for Linux is still in beta, and is free until it will be in beta version. After that it will cost a one-time fee of $9.99 for each account that you want activate.
Grive: is a good open source solution, but has some limitations, as no-automatic syncing of files in background, even though is possible make up for this problem running grive automatically, every 10 minute for example, using cron.