I landed here because I was looking for a way to merge in an external merge editor (KDIFF3) but start the merge from eclipse. I wasn't satisfied with the answers provided above. So here is ho to configure kdiff3 as merge and diff editor for SVN in eclipse:
go to Windows -> Preferences → Team -> SVN -> Diff Viewer
Add a new config (add button):
Extension or mimetype: * - if you wish you can specify different mimetypes for different editors, I didn't need that thus the alquantor.
Diff:
Program path C:\Program Files\KDiff3\kdiff3.exe (or wherever you have your merge editor - sry for the windows path, feel free to add a linux version in the comments or edit this answer.)
Arguments:
${base} ${mine} ${theirs}
Merge:
Program path C:\Program Files\KDiff3\kdiff3.exe
Arguments:
${base} ${mine} ${theirs} -o ${merged}
This will probably work as well for other merge editors, but with a different argument syntax (figure it out an let us know :) ).
The usage is as usual (team->edit conflicts) for merging and compare->foo for the diff view.
Cheers