Snapshot Server is a small rails application that serves OSM data using standard map calls. It is based on the "pgsnapshot" schema created by Osmosis, so is a much simpler and more lightweight way of serving data than installing a full rails-port instance. It was written by Gravitystorm in order to serve 3rd-party vector data as background layers to editors such as Potlatch 2.
An installation of the app can handle multiple .osm files at the same time, and treat them as separate projects.
Written in Rails 3, it's available at https://github.com/gravitystorm/snapshot-server. Patches and forks welcome.
As well as serving normal map calls, it's possible to track the "status" of individual entities and allow users to change the status. For example, this allows volunteers to work through a set of third-party data, marking each feature as "complete" when it's been dealt with.
The snapshot server and associated work in Potlatch 2 was commissioned by CycleStreets to help merge cycling data from an external government datasource into OSM. They made a blog post about the process which has some screenshots.
For interest, here are some projects using the SnapshotServer