This is weird. I've used WordPress for a long time and I've never run into this issue.
My Plugins directory is in the right place and its permissions are set properly. However, the contents seem to have no bearing on the list which shows up on the Plugins page in my WP Admin! Uploaded plugins don't show up and, when I remove plugins from the directory, they persist on the list.
Get this: I just renamed my Plugins directory (to "lugins") and the Plugins page still seems to operate normally!
Anyone have any ideas?