Scangauge II is the absolute best you can get.
Ultragauge is a close second; it lacks some bells and whistles of the SG II, but for a casual user it is still more than enough, and its significantly cheaper. Many people also find it's readout clearer and more attractive than the SG II.
Then there is the mpguino and superMID; it's almost impossible to find a superMID nowadays, but you can easily find an mpguino. If you are a DIY type with elementary knowledge of circuit analysis, logic design, and C programming, you can also build one yourself using an arduino and simple electronic circuit parts.
Finally, if all of the above are too much of a hassle to get/to expensive to get, you can find a decent vacuum gauge and use it to fine-tune your throttle control; it gives much, much less information than any of the above devices, but it's still better than nothing.
*edit: forgot to add: a compatible OBD2 dongle can be used with the Torque app (Android) or REV (iOS). Personally I'm not a big fan of either, but its an option nevertheless.