Bunk beds are surprisingly expensive if you get a sturdy one, and the cheap ones feel pretty flimsy. This one is very easy to build from off-the-shelf lumber, and is rock-solid. Well, at least as solid as your house. This bed was made for my two small boys, but it's strong enough for anyone... as the second picture attests. Try doing that with a regular bunk bed! (actually, don't. It's not very comfortable. Lie on it instead).
Note: this bed worked so well I made two more, this time supported with a combination bookshelf/ladder.