However, there is a small collection of oss Ada libraries out there.
Anyway, Ada has some other amazingly cool features. The concurrency primitives it offers are very cool, lets you make some much stronger guarantees about the interactions between threads than any other language I've seen. For example, you can define rendezvous sections, which if memory serves, are pieces of code that are guaranteed to only be run once both threads participating in the rendezvous and neither thread can leave the section until both are ready.
You're right about the concurrency. Ada has a bunch of stuff like that built into the language since 1983.
The particularly cool toys I like are SPARK (a formal verifier tool) and stackcheck - tells you exactly how deep in the stack your code can possibly go. (Yes you have to annotate cycles.)