show version -- displays the configuration of the system hardware, the software version, the names and sources of configuration files, and the boot image
show processes -- displays information about the active processes
show protocols -- displays the configured protocols; shows the status of all configured Layer 3 protocols
show memory -- shows statistics about the router's memory, including memory free pool statistics
show stacks -- monitors the stack use of processes and interrupt routines and displays the reason for the last system reboot
show buffers -- provides statistics for the buffer pools on the router
show flash -- shows information about the Flash memory device
show running-config (write term on Cisco IOS Release 10.3 or earlier) -- displays the active configuration file
show startup-config (show config on Cisco IOS Release 10.3 or earlier) -- displays the backup configuration file
show interfaces -- displays statistics for all interfaces configured on the router

show arp
show clock
show hosts
show users

CDP - runs over a Data Link Layer that connects lower physical media and upper network layer protocols
show cdp neighbors
show cdp interface -- display the values of the CDP timers, the interface status, and the encapsulation used by CDP for its advertisement and discovery frame transmission. Default values for timers set the frequency for CDP updates and for aging CDP entries. These timers are set by default to 60 seconds and 180 seconds, respectively.
show cdp entry {device name} to display a single cached CDP entry.
show cdp neighbors detail to display this information as well as information like that from show cdp entry.