Linux for developing Ada real-time applications
GNAT/Linux: catalog

Development
Compilers and libraries
GNAT
GLADE (distributed systems annex)
GtkAda
ASIS
AdaBroker (100% Ada CORBA solution)
Environment
Emacs, XEmacs, GLIDE
GDB, DDD

Other advantages
Low cost solution
Easily upgradable, customizable, duplicatable