From splash pages to general panels, floating heads were utilized more frequently than you may have imagined. Whether representing spirits, dreams, past loves, narrators, hidden identities, links to the negative zone, or roll calls - this artistic flourish is quite unique to the comic book medium.
Spider-Man sure had his share of examples, as did Captain America. Captain Marvel and Firestorm embedded the technique into their storytelling of linked characters. Great artists like Sal Buscema, Mike Zeck, Pat Broderick, Gil Kane, Jim Starlin, and John Byrne used the style frequently.
There are more examples than I could possibly find, but I tried to capture a representative cross section. Take a look and feast your eyes...today there is plenty to enjoy! Cheers!