Are there any comparable tankless water heaters to the Navien NR-240A?
The NR-240A is a condensing tankless water heater. Condensing units are more efficient because they are able to capture almost all of the heat content from the fuel. Currently (November 2010), there are 5 manufacturers that offer condensing units. They are (listed alphabetically):
Examples of these types of units:
Takagi TH-2 - currently not "generally available"
Be aware of comparing different units and manufacturers. There are different features, different abilities to heat water, different warranties, venting methods, etc. As for which unit or manufacturer is the best, I cannot say.