Ranked in these QuestionsQuestion Ranking
Pro Only needs to be installed on one of the machines
The client just needs a web browser. Even mobile devices can use it, both for uploads and downloads.
Pro Free and Open Source
You can run it from a USB stick or something.
Con Server must be run on Windows
It does run properly on WINE, so you can make it work on Linux. It's just harder to install. The client, on the other hand, can be anything with a web browser.