Here are two quick macros that I have been using for merging via the external tool Araxis Merge.
Calls Araxis Merge on the current open file in two flavors:
mergeitsolo: Just the buffer in a single pane.
mergeit: Diff with the lhs and rhs being the same buffer name.
I do a lot of work with multiple Perforce streams, but you can extend this to other goodness.
- Jeff