Times change and I advise you to check this tip on http://www.alandstrafiken.ax/eng/index.htm

When we went, the inter-island ferries [NOT Viking or Silva] were free for pedestrians, cyclists [bikes too.] They were also free to motorists who were staying for a minimal number of nights on islands between the main island and the Finish mainland.

Mariehamn urban buses were also free but not the buses from Mariehamn around the island.