Nethack is arguably the best game available for BE.
Nethack is a huge, highly addictive dungeon RPG game. It's free and it's been in development for more than a decade, if not two.
There are two versions of nethack that can be played on BE: palm sized version and ppc version. For ppc version you have to use the ugly hack described below; palm sized version works perfectly but lacks the button that toggles full map view and zoomed view. Instead, you have to go to map menu and click on full view - I believe this is too much work for something you need to do at least couple of times on each level. Nethack is a long game, and you often have to go back and forth between levels, therefore I think, despite the kludge, ppc version is much preferable.
Getting nethack to work on BE is a little tricky. Naturally, you need BeShell or some other advanced shell. The rest of this page applies to BeShell but may be true for other shells, as well.
You need to install through ActiveSync as usually. Now comes the tricky part: you need a hardware or shortcut button that launches the list of tasks, before you proceed. After you set it up with BeHotKeys in utilities menu, you can go ahead and start Nethack. It will ask you for your name, which you have to enter; after that you will see a black screen with a keyboard on the bottom and one highlighted cursor square. Now is the time for you to use that button that you set up - press it and you will see a tasks list with two nethack tasks running. If you try to switch to either one of them, you will still stay with that black screen you saw first; even if you try to switch to desktop you'll still be seeing that black screen.
Here's what you have to do now: kill one of those nethack tasks. It will go straight into the game window, skipping all setup questions and assuming random choices for all of them - still better than nothing, right? Now you're good to go.
Note: nethack is a fairly complex game with a huge number of commands, it's not anything like your typical commercial game - but it pays to learn its ways. You might say that playing games is a waste of time and I may even agree, but if you're playing any game at all, nethack just might be the most intricate, addictive and complex game universe there is. Every new game is unlike any other.
My point is that you have to take a look at the help file before playing. One little hint: you need to eat monsters you kill to stay alive, but some of them are poisonous.. if I remember right, kobolds and snakes are, but almost all of the rest are edible. This tripped me up when I first started playing: I got it into my head that all monsters may be poisonous and I couldn't last on food rations I could find. Good luck!
Note: if you go into memory boost mode, you will not be able to type in your name and you won't get past that screen, because there's no keyboard on screen. I thought this bug may be caused by insufficient memory and, well, it may be but there's no way to check, I guess..