There are some means for users snooping, some are already on a regular system, the others border on the malicious and usage could well break your box (or open security holes).
Lastcomm will give you all commands entered in the console (man lastcomm),
then there's the shells' history file (~/.bash_history for bash) or you could execute "script" from ~/.bash_profile, but both history and script are easy to thwart.
You can use tcpdump to log sessions.
If you want to log commands from a pty/tty then appcap, ttysnoop, linspy or a kernel module called "exec.o" should do.
If recompiling the kernel is no problem, then the Grsecurity patches also provide a way to log user activity.