In Visual Studio, we can "Add as link" to add a link to a file in another project in the solution.
Is there any way to do this for entire folders, so that an entire folder in project A will be visible in project B, without the need to manually link to new items in that folder?
Bust out the shell and add a symbolic link.
runas Administrator then
BAM symbolic link!
/d specifies directory link.
Works in Vista on up out of the box. Can be backported to XP.
Documentation here: http://technet.microsoft.com/en-us/library/cc753194%28WS.10%29.aspx
For those not familiar with symbolic links, it's essentially a pointer to another file or directory. It's transparent to applications. One copy on disk, several ways to address it. You can also make a "hard link" which is not a pointer to another address, but an actual file ID entry in NTFS for the same file.
NOTE: as stated in the comments, this would only work on the computer where you created the symlink and wouldn't work in a Version Control System like git.