Proc file system

Read-only files:
/proc/ Information on running processes (ps, top)
/proc/cmdline Boot time options
/proc/cpuinfo processor information
/proc/mdstat software RAID information (mdadm)
/proc/meminfo system memory usage (free, vmstat)
/proc/swaps system memory usage (free, vmstat)
/proc/modules dynamically loaded modules (lsmod)
/proc/mounts mounted filesystems (mount)
/proc/net network activity and configuration (ifconfig, netstat)
/proc/partitions block devices known to the kernel
/proc/version version of the Linux kernel (uname)

Read-Write entries:
/proc/sys/kernel/hostname System hostname
/proc/sys/net/ipv4/ip_forward IP Forwarding (on or off)
/proc/sys/vm/drop_caches Writing a 1 forces the kernel to free up some memory from caches.
/proc/sys/vm/swappiness Indicates how aggressively memory will be swapped out to the swap device (number between 0 and 100).