Right click on the resource in the Eclipse Project Explorer/Package Explorer > Open With > Other ... A dialog comes up and shows you the list of internal editors and external applications available on your machine.

It might be easier to hear about what types of files you are interested in :-)

Eclipse comes with editors for text and java for example. It can open any document using a system editor as well. There are many plugins out there which would provide additional support for particular types (javascript...).

Finally you can keep using your favorite editor/application to edit and save files. Then use RTC Eclipse client to check-in these files.

HTH,
Chrix

Is there a list there of what types of files can be edited directly thru the RTC eclipse client ?

As Chrisx stated types files, which can be editen in RTC client, depend on the plugins you have installed.
You can have a look at the following option: Window->Preferences, then in the tree on the left go to General->Editors->File Associations. Here you can associate editors for your file types, moreover when clicking 'Add' button in the 'Associated editors' section you can see what editors are available in you RTC Eclipse client.