SAP db. Source was recently made available. They've got an interesting licensing scheme - there's no license fee, and it is open source. If you run a SAP environment (i.e. the biz tools that use SAP db or Oracle as a backend) you have to have a valid support agreement that covers SAP db in order to use SAP db.
Since in essence the only commercial folks running SAP db that are worth anything to SAP are running the SAP business tools, this scheme lets SAP opensource the db without losing any appreciable revenue.
I don't know much about SAP db but did download it a couple of nights ago and intend to investigate it before too long. Don't know details about locking, etc.
Interbase. I don't know if they've gotten the political mess straightened out or not. Has a multi-generational storage mechanism very similar to PGs, which provides row-level locking. Unlike PG, though, IB reclaims disk storage without having to take explicit action (i.e. VACUUM in PG). PG 7.2 will reclaim storage, too (at least that's the plan).
MySQL has been married with a new storage backend, InnoBase, that provides full transaction support, row-level locking, write-ahead (redo) logging, etc ala Oracle or Postgres. It's a traditional overwriting storage manager that rolls back transactions via the redo logs. Has limitations (one familiar-sounding restriction: data limited to 8KB rows). The plans are to get rid of them, of course. The author's not part of MySQL so there's hope that it's a solid, well-done piece of work.
