The pgrep utility is actually not a program developed specifically for FreeBSD - its history section says:
"The pkill and pgrep utilities first appeared in NetBSD 1.6. They are modelled after utilities of the same name that appeared in Sun Solaris 7. They made their first appearance in FreeBSD 5.3."
... but I have a feeling it's still one of the underused utilities. Again, it is best to explain what it does with an example:
renice 10 `pgrep make`
That command will renice all "make" processes, such as those started by the buildworld process (the point being that all the compilers and other utility processes will inherit this niceness value).
This is probably the simplest usable example of using pgrep, but the utility supports a large number of parameters with which various sets of processes can be selected, such as those belonging to a certain user, or having a certain parent process, or from a certain jail, or even inverting the query (i.e. querying processes NOT from a certain set). The name argument to pgrep is actually a regular expression pattern, which can be further used to find very specific processes.
It's one of those utilities which can save a lot of time if used properly.