I use named branches in Mercurial.
In doing so I have created one branch called playground where I can try out various wacky experiments. I never intend to merge this b
In addition to the excellent answer above concerning phases, you can also specify 'default-path' (in the [paths] section of your .hgrc) to refer to the local repository:
[paths]
default = ...
default-push = .
This will cause all outgoing changesets to be compared to the specified repository. In this case, comparing outgoing changesets in your local repository TO your local repository results in nothing to push.
You can still pull/update/merge from the main repository, but no push will ever send anything back to that main repository.
If you work on multiple machines/repositories, you can set one up as described above, and configure the others to specify the 'default' path to point to the server that pushes to itself. In this way, the other machines can push/pull to your local central repository, and these changesets will never escape your carefully configured collection of repositories.