OpenCover is the successor of PartCover and actively developed (as of feb. 2013).
Find it on GitHub or install as a NuGet.
From the creator of both products:
NOTE: PartCover is now in Maintenance mode (fixes only) all new development is being done on OpenCover which now has 32/64 bit support and also supports branch as well as sequence coverage.
You may also wish to use ReportGenerator to get human-friendly reports.
Available on Codeplex and also as a NuGet.
As far as I can tell, it's the reporting part that lags behind commercial offerings, lacking custom aggregation/drill-down abilities, but it's certainly usable.