http://code.google.com/p/angdroid/Angdroid, aka Angband for Android. There are four downloads, available from the above site or via Google Play:
* Angband-3.3.2.4.apk is intended for classic, "vanilla" (stock) Angband in versions considered stable; it includes three versions: 3.3.2 (current stable), 3.0.6 (before a number of significant changes), and 2.4.frog-knows (which was the basis for the DOS version among other early ports, and how many people learned years ago).
* Angband-3.4.b1.apk is the current beta / bleeding-edge version of vanilla Angband.
* AngbandVariants-r260.apk contains three classic major variants; these are generally options that go beyond merely mods into significantly expanded gameplay. Includes 3 versions: ToME 2.3.4 (my favorite, on the one hand with an overworld and a plot, but also with all sorts of zany over the top classes and races), Sangband ("skills Angband", with a neat skills system), and Steamband (a full Steampunk re-make).
* RoguelikeClassics-r260.apk contains three retro/classic early roguelikes: Rogue, Larn, Moria.
All of these are free-as-in-beer, and mostly free-as-in-speech (libre); note that their licenses predate the EFF, GPL, etc. and are not always 100% compliant with modern FLOSS licenses (see
Angband and its license: a history. After a major effort to find people from decades ago, 3.1.1 and more recent versions are generally considered GPL clean.
I've played 3.3.2 vanilla and ToME 2.3.4 on my Droid 3 smartphone; it's fully playable, but the screen is tiny enough you need good eyesight, and it'd be somewhat less easy to use on a phone screen if I didn't have the hardware keyboard. Any actual tablet should do fine.
If you like some of the crazy weirdness of DF, I'd suggest trying ToME; note that some of the more eccentric starting options ("Always Unusual Rooms" and starting as a Lost Soul or Death Mold in particular) are analogous to starting DF on a haunted glacier, and not suggested for beginners (but can provide considerable Fun if you're not afraid of dying horribly within a few keystrokes).
http://code.google.com/p/nethack-android/Nethack 3.4.3 is also available, from a different group; not my favorite roguelike but should be similarly playable if that's your thing.