Normally, artifacts have a URL of the form:

[base URL]/download/[project key][plan key]/artifacts/build[build number]/[artifact name]

But you can obtain a permanent URL that will always link to the most recent version of an artifact by replacing the second last section with "latest", so that it becomes:

[base URL]/download/[project key]-[plan key]/artifacts/latest/[artifact name]

We should document this feature.