Google has Bigtable and Datastore. Bigtable can be thought of as an events database, and Datastore is entities. Both very very compelling for DynamoDB users.
Google's Lambda equivalent is currently in Alpha. That said, there are bits like Cloud Logging and AppEngine that offer unique-to-Google functionality that's somewhat comparable.
Both technologies are clearly very compelling to users.
This is a bit outside of my expertise, but that page mainly points out that DynamoDB supports 3 more language-specific SDKs, while lacking ACID and what appears to be native cross-region replication (through Lambda). Until recently, Datastore was coupled with AppEngine that might explain it.
Datastore and BigTable looks pretty interesting, and I am having a hard time figuring out when to use which (apart from Datastore's ACID support) as both seem a good enough fit for most workloads.
You're correct in that Datastore does provide more general transactions (Bigtable only provides single-row transactions); Datastore also provides indexing and search (Bigtable, like HBase, has a single index: the row key), and Datastore is synchronously replicated cross-zone and cross-region.
I don't think there's an equivalent to Bigtable anywhere. It's so badass it's not even funny - take a look at [0] - a small team of engineers took O(weeks) to build a system that clocks 56 million qps (and these aren't simple bits - 56 million FIX protocol trades and orders).
You can think of Hbase as a clone of Bigtable, if that helps. Bigtable is actually exposed through the Hbase API. /u/mbrukman here is the PM for Bigtable.
Last I checked the pricing was very different; nodes for BigTable, reads/writes for Datastore. BigTable is more cost effective for large workloads, but it requires buying a minimum of three nodes, which is ~$1,500/mo total. If you're small potatoes it seems infeasible. Happy to be corrected if I misread.
Google's Lambda equivalent is currently in Alpha. That said, there are bits like Cloud Logging and AppEngine that offer unique-to-Google functionality that's somewhat comparable.
(Work on Google Cloud)