I'm about to release the initial version of hg4idea plugin for Mercurial integration.
For now it provides some basic integration: add file, remove file, rename, move, commit, view diff, view history.
More to come in the next version.
More details here: http://code.google.com/p/hg4idea/
Please help to test plugin on Windows and Mac, use these testcases for minimal testing
If you have any comments or would like to add some testcases, open an issue or comment on testcase
If you find bugs, please log them here: