The Update Entries plugin lets you to easily update rows in any database table — whether within ExpressionEngine’s database or in any database to which you have administrator access.

Together with the Insert Entries and Select Entries (formerly External Entries) plugins, it’s an easy way to synchronize EE with other MySQL-based systems, such as a commerce platform. The other system can send a form to the page containing the Update Entries plugin, which can update both EE (such as changing the status of a payment from pending to paid) and/or another system in turn.

For more information, see the docs.

The plugin costs $15 per EE install and is free to download and try (requires registration at Engaging.net) before you buy.

Update: Now compatible with both EE v1.x and v2.x.