Leveling is *not* the same as compressing.
A leveler is basically an automatic gain controller. A VCA in other words, but one that automatically rides the gain to keep the average volume consistent. It's like a hand that rides the fader, and that of course is a bit different than compression.
The following could be said about compression: "compressors make the loud sounds quieter and the quiet sounds louder". While that could also be said of the leveler, there are some differences. The compressor has ratios (i.e. 3:1, 6:1, etc.) that affect the amount of compression, something not used in a leveler. Compressors also have threshold settings, which allow only a portion of the signal to be processed. For example, in a compressor only the loudest signals could be compressed, leaving untouched anything below the threshold.
So while both levelers and compressors do affect dynamics, they approach it differently and yield different sounding results.
The Aphex unit mentioned earlier in this thread is called
the Compellor. I have one (modded by Audio Upgrades) and it is indeed a very handy unit to have around. It can be used as a straight leveler or straight compressor. However, the ratio of leveling to compressor can be adjusted so that you could have 50 percent leveling and 50 percent compression, for example. What's amazing about the Compellor is how invisible it is, properly set you really can't hear it working.