If you don't know NuGet, then you are missing out. NuGetis a package downloader ala gems in the ruby world. There is also the package explorer by dotnetjunky to browse through the packages. This is fun and it has some stats too.
Looking at the stats we can see which opensource projects are very popular and perhaps worth a look if you don't know them yet.
These are the first 15. And I know most of them. Pretty sure that the number one position of the EFCodefirst package is due to people wanting to try it out and people using it for demos perhaps.
So download package explorer do file -> open from feed and have fun. Double click on a package to see what metadata it contains.
Perhaps now you can pickup some opensource packages you never knew existed.