Tools like uptime and free (or freecolor on FreeBSD) are terrific. But, what if you want to get stats from the past few hours or days? You could rig up a shell script to capture these tools to a log file.
These commands efficiently maintain a history of your server performance and allow you to get to the stats after the fact. This is wonderful when you think you've got a server performance issue, but don't have much in the way of stats to back up your hunch.
My suggestion: make sure the sar command is installed *before* you need it.
On Linux I run:
yum install sysstat
To get access to the sar command. On FreeBSD I followed the helpful instructions here.