Crowdsource is described as 'Answer fun and easy questions that make a huge impact and level up with achievement badges and perks along the way! Your voluntary contributions make Google products better in your part of the world'. There are five alternatives to Crowdsource for Online / Web-based. The best alternative is Museum Metadata Games, which is both free and Open Source. Other great apps like Crowdsource are MetaData Games (Free), Book Tag (Free), ARTigo (Free) and Ships Tag (Free).
Metadata Games is a digital gaming platform for gathering data on photo, audio, and moving image artifacts. The platform entices players to visit archives and explore humanities content while contributing to vital records.
ARTigo is an online game with the aim to supply artworks with tags. The same artwork is simultaneously shown to you and a co-player. The game now consists in describing the artwork accurately with tags.