Mipui is an online map editor based on grids, which focuses on simplicity and collaboration: all editions appear in real-time for other viewers. It intends to be used for role-playing games such as D&D and other board games, useful for both before a session as a mapper and during a session as a virtual table. All maps are stored indefinitely in the Cloud. So, you can use Mipui if you need an alternative for Roll20.