What other space applications, projects and agencies are using the Ada programming language?
In the US, old stuff such as the Delta rockets, the Atlas rockets, and the TDRSS ground terminal (but this is being switched to C, C++, C#, and Java). There might be new development, but it's mostly stuff you (and I) cannot know about because it's classified. New development is mostly in mainline languages such as C, C++, C#, and Java, and also specialized tools such as Matlab Simulink.
One reason for the shift away from Ada was that the Ada mandate came out too soon. Some projects that foundered because of lack of tools switched to C. The managers of these projects remember that Ada experience. The memories are not fond.
Another reason was demographics. Finding Ada programmers, good or bad, has always been a bit tough. It's much easier finding C or C++ programmers, and this gives managers the opportunity to select for quality.
Yet another is that many of the developers of tools that initially targeted Ada (e.g. Rational) found that their tools worked quite nicely with other languages. Because of the demographics, improvements and new features were first made to the mainline language versions before extending them to Ada. This added even more impetus to switch from Ada.
Ada appears to be more widely used in Europe than in the U.S. One reason is that the chief architect of Ada was Dr. Jean Ichbiah at CII-Honeywell-Bull in France. The Ada mandate was applied in Europe as well as in the U.S., but the timing was a bit better there. The tools had matured, and many of the tools were developed in Europe by the company founded by Dr. Ichbiah. A recent example is the flight software for the GOCE satellite.
A not so good example is the flight software for the maiden flight of the Ariane 5 rocket. This is perhaps the most infamous of all software errors. Ada has a number of safety features built into the language. (Compare with C, where the standard has twelve pages that summarize the undefined behaviors built into the language. Compare with C++, where the undefined behaviors are so numerous that they don't even list them.) Those Ada safety features can have a significant performance cost, and because of this, Ada also supplies the ability to selectively disable those safety features.
In the case of Ariane 5 flight 501, the software engineers reused Ariane 4 flight software, where safety features relating to overflow in some of the GNC software was disabled. The Ariane 5 had considerably more thrust than did the Ariane 4. This increased acceleration made the accelerometer data overflow on conversion to an integer. This in turn resulted in hardware exceptions, which in turn made the GNC software come to a halt, which in turn resulted in the destruction of the vehicle.