Synergy is quite good if you can make it work reliably with your setup. When I used it with my Mac as the server and my PC as the client, I ran into a bunch of issues such as modifier keys getting (virtually) stuck. Then I switched it around and made my PC the server and my Mac the client and a different set of problems arose. Luckily, there is an active discussion group and it's open-source, so if you were really keen on making it work, you could.
XFCE itself is still written to be modular and lightweight (although the definition of lightweight has changed over the years). Most of its components can be installed separately with few dependencies. Distributions like Xubuntu and Fedora pile on tons of packages to fill up the CD images and present a nice, full-featured desktop to new users. After the Gnome3/Unity upgrades I switched to Xubuntu and just rip out about 50% of the packages after doing a clean install.