Indeed, Aaron B., Roam isn't local. Mirna L., Roam is a block-centric outliner and as such the main similarities with Obsidian are the internal links and the backlinks, the rest is quite different. Basically, Obsidian is similar to Foam and Dendron, while Athens and Logseq are very similar to (and strongly influenced by) Roam. Athens is collaborative. Logseq is local and MD so it is interoperable with Obsidian with the exception of block references and queries.
Roam is paid and not cheap. If you are looking for free cloud-based tools like that, try https://scrapbox.io/. It predates Roam and is much simpler but quite good for individual or team use. And there are plenty of other options, including https://zenkit.com/en/hypernotes/, and soon to be released https://unigraph.dev. Many Obsidian users highly recommend https://www.craft.do .
In all of them, the graph is very simple, either linking only pages or linking also blocks but you can't specify the type of relationship. If you a looking for that kind of graph tool, there are not many. I know only https://kanopi.io and https://atomgraph.github.io/LinkedDataHub/ which was already suggested. The former doesn't have standard querying (SPARQL) but has features for unstructured content, the latter is full-blown RDF platform but to my knowledge, it only supports structured data.
