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There is nothing called best open source tool in maintaining the test cases, because every test management tool has one or the other best feature which are not present in others test magt tools. And it also depends on the type and size of the organization.
I could suggest you to go through this link:
I hope this info will serve your purpose.
I use Bugzilla for maintaining the bugs and features list and an Excel sheet for maintaining the test cases.
P.S: You can also read the discussion on "The Metrics" for more details.