Depending on what's "good" for you,
there are some choices:
Running natively on FreeBSD:
https://cgit.freebsd.org/ports/tree/games
As hbsd already pointed out, there are several versions/clones/variations of unix's classic nethack,
even with graphics.
Many like
0ad, a clone of MS' Age of Empires.
Recently I just checked out
minetest, some kind of openworld.
I also played a bit
flare, which is a hack'n'slash kind like Diablo.
For a while I played
endless sky, a 2D space explore, travel, trade, and combat game with lots of very nice fotos (install the high resolution graphics version is a must!)
I took a short peek at the space adventure sim
pioneer, which is quite large, and very complex (too complex for me).
opencity, lincity, or simutrans are also worth a try if you like to build.
There also is a clone of Master of Orion 3 available,
freeorion.
...
There's quite a choice of games available, partially real good ones.
Point is, most don't provide the nice graphics you are used to by big commercial providers,
and some are not what I would call "finished", or "finetuned."
For me also some offer too many options to be set.
I lose myself fumbling with options between "cheating" and "way too hard" instead of playing the game, so losing interest in it quickly.
However:
You have to give some a shot for yourself.
The list were way too long to give even short revisions for each,
which would also be very subjective, of course.
Running not natively on FreeBSD you have more choices:
Using wine, dosbox, or a virtual machine like VirtualBox, you shall be capable to run any software from other systems.
So you shall be able to play any game on FreeBSD.
So far the theory.
In real life there are some strings attached:
Besides you may not observe the smooth running of current 3D games like they were running natively,
you may get some probs to get them even running, especially new games, especially with 3D.
In theory wine would be the best choice to run any Windows software,
but in reality wine is tricky to be set up to have the sw run smoothly.
You may face a lot of "missing library", errors, and other config issues.
winecfg and winetricks may help with some but not all programs.
Better first learn about 32/64bit, WINEPREFIX, and run the sh .386something script the wine installer mentions,
before even bother to try some serious stuff with wine.
For myself I run
factorio under wine (but cannot connect to the website, so no mods but vanilla only), and with larger plants the game slows down like it were running on 10y old hardware.
On the other side I have XP running in VirtualBox to play
Master of Orion 2 (much better than 3), Heroes of Might and Magic 3 (4 & 5 are not running because of 3D issues) and Diablo (1).
Summery:
Best chose old games (>20y).
Proven to be good games, least issues.
Workaround:
Have a second partition/drive/machine just for games,
and play the games natively under Windows.
