Are there any freely available Ada plugins eclipse. Eclipse is my main IDE and I occasionally need to read and modify some Ada, having it all in the one IDE would be ideal.
If all you need is syntax highlighting, then maybe EclipseColorer will do the trick for you? It's a general-purpose extensible syntax highlighting engine that has definitions for Ada out of the box.
For Eclipse, AdaCore distributes and maintains the GNATBench plug-in, though I've never personally used it. I believe it is not GNAT specific, so it may still be of value even if you're using a different compiler--but I could be wrong on that :-)
If you're using the GNAT Ada compilation system, you've got a fully Ada-aware IDE in their GNAT Programming Studio (GPS), which is what I've been using ever since it finally stabilized a few years ago. It's got the jump-to-definition, get references, specialized search, etc., capabilities that you'd expect.
These, as well as the rest of AdaCore's GPL Ada development environment and tools, are available from AdaCore's Libre website. Download page is here.