I have a build configuration which deploys my code to a machine. Depending on which machine I am deploying to (e.g. dev/uat/prod), I need to run as a different user.
Ra
The RunAs plugin combined with TeamCity 7's new Typed Parameters will let you make the password a "typed" parameter plugin.
Then, when it's entered at the Run screen, it will not be visible in the build history.
EDIT: Much later, as covered in the comments: You probably don't want to do this. Consider having separate pools which run as different users, and parameters to specify what builds are supported by what pools.