Oh I love bagpipes! But that's a whole 'nother thread.
Acoustic guitar blends with just about anything. Great with fiddle and banjo, anything else acoustic. I'm a real fan of Gaelic music, and I'm remembering one cut by Ossian that combines guitar, fiddle, harp, and whistle. I get goosebumps just thinking about it.
Electric: depends on the style, but (1) organ and (2) sax come to mind.