In comics, I'd say a lot of inkers fall into this category. The most prominent in my mind would be Terry Austin on X-Men. I can't even imagine that book without his beautiful fine line work. Nowadays when I think of the creative team for that incredible run, I think Claremont-Byrne-Austin, because I have no doubt that his art work helped define the look and feel of the book every bit as much as John Byrne's pencils.
Another great inker has to be Joltin' Joe Sinnott, who gave the Fantastic Four a sense of continuity regardless of who was drawing the book. He's a strong inker, yes, but a good one.Sinnott deserves to be included in any discussion of the FF from the 60s, 70s, and 80s.
A few weeks ago we discussed the original Star Trek TV series, and the bad blood between star William Shatner and some of the supporting cast. While the show undoubtedly revolved around Shatner, Leonard Nimoy, and Deforest Kelley, would it have been quite as fun or had the same flavor without the efforts of Nichelle Nichols, James Doohan, George Takei, or Walter Koenig in their respective roles? I have a hard time imagining anyone else but Doohan saying, "I'm givin' her all she's got, Captain!" or anyone else doing that excruciating Russian accent than Koenig.
Recently I saw a documentary on the Eagles, and was struck by how much band members Don Felder, Bernie Leadon, and Joe Walsh brought to the group. So much is made of Glenn Frey and Don Henley as the songwriters of the band, but many of the big hits for the group were developed out of riffs that those three guitarists came up with. Where would the band be without them?
There are a ton of other unsung heroes out there - so start naming them!