One possibility is /proc mounted with either hidepid=1 or hidepid=2. This mount option was added in latter Linux kernels and back ported sometime around CentOS 5.9 and 6.3.
Mount options The proc filesystem supports the following mount options: hidepid=n (since Linux 3.3) This option controls who can access the information in /proc/[pid] directories. The argument, n, is one of the following values: 0 Everybody may access all /proc/[pid] directories. This is the traditional behavior, and the default if this mount option is not specified. 1 Users may not access files and subdirectories inside any /proc/[pid] directories but their own (the /proc/[pid] directories themselves remain visible). Sensitive files such as /proc/[pid]/cmdline and /proc/[pid]/status are now protected against other users. This makes it impossible to learn whether any user is running a specific program (so long as the program doesn't otherwise reveal itself by its behavior). 2 As for mode 1, but in addition the /proc/[pid] directories belonging to other users become invisible. This means that /proc/[pid] entries can no longer be used to discover the PIDs on the system. This doesn't hide the fact that a process with a specific PID value exists (it can be learned by other means, for example, by "kill -0 $PID"), but it hides a process's UID and GID, which could otherwise be learned by employing stat(2) on a /proc/[pid] directory. This greatly complicates an attacker's task of gathering information about running processes (e.g., discovering whether some daemon is running with elevated privileges, whether another user is running some sensitive program, whether other users are running any program at all, and so on).Another possibility (found by the poster and added to this answer as reference information) is grsecurity which has a feature for hiding other users' processes from unprivileged users as part of it's file system hardening.
Hide other users' processes for unprivileged users
While the upstream kernel now provides a mount option for /proc to hide other unprivileged users' processes, grsecurity goes beyond this by hiding such information by default, hiding additional sources of sensitive information provided by the kernel in /proc, and hiding private network-related information of all users. Not only is the networking information a violation of the privacy of other users on the system, but it has also been useful in the past for TCP hijacking attacks.