Think funding of Open Source projects are important? Then, here is a great list to start to do it from!

GNU Operating system

Open Source Initiative

Free Software Foundation Europe

SF Conservancy

Libre Office

F-droid

GIMP

Gnome foundation

KeePass

KeePassXC

NoScript

Synergy

WinSCP

VLC

phpMyAdmin

AdBlock Plus

OpenSCAD

OpenELEC

OSMC

Syncany

JDownloader

OpenWRT

MPlayer

LibSSH

Awesome WM

BitlBee

FlexGet

Pitivi

Haiku

Pandoc

Kallithea

Twisted

MariaDB

Some others we think you will also like

Wikipedia

EFF

Open street maps

The Internet Archive

W3C Validator

Duck Duck Go

Project Gutenberg

TORServers

SQLFiddle

JSON Generator

Can I use

Cory Doctorow

Terms of Service; Didn’t Read

OMG Ubuntu

EtherPad

Know any more great FOSS projects using Flattr, drop them in the comments!