DC One Million - DC Comics. Featuring a Justice League from the very, very far future coming back through time to meet the people who founded their heroic dynasties; this series also sent DC's most popular characters into this same far future. Orginally published as a cross-over miniseries, this story has since been collected into an easy to find single volume.
The Guardians of the Galaxy - Marvel Comics. A team Marvel has tried several times to build a monthly series around, none of the attempts have lasted particularly long. A good counterpoint to the happy-shiny vision of the future portrayed in DC Comic's The Legion of Super Heroes. Not currently published, and difficult to find, but the back issues are generally cheap when found.
Green Lantern - DC Comics. - Set in "modern day" as opposed to far-future, but during serveral time periods of it's publication this series exemplified the interstellar superhero.
The Legion of Super Heroes - DC Comics. This is the most well known of the far-future superhero comic, and is a great source of adventure ideas. Features young superheroes in an optomistic (near Utopian at times) interstellar federation. An on-going series available each month. Early issues are also available in collected volumes from the publisher.
L.E.G.I.O.N. & R.E.B.E.L.S. - DC Comics. Like Green Lantern, this is not far-future, but the stories were set against an interstellar backdrop and featured a cast of characters made up of superheroes from human and non-human varieties. Neither series is still being published, but back issues can be found and are generally cheap.
Humanx Novels - Alan Dean Foster - One of the best examples of far future, multispecie, interstellar civilizations put to paper. Bloodhype, Flinx in Flux, For Love of Mother Not, Mid-Flinx, Nor Crystal Tears, The Tar-Aiym Krang, The End of the Matter, Orphan Star.
Star Trek - Paramount. Novels based on the popular Gene Roddenbery creation.
Cosmic Safari - Discovery Channel / The Learning Channel. If life evolved on other planets, what would it look like? In this series, TLC leads us on a safari through our own world and into the deepest regions of space. Great resource for building alien races.
Star Trek - Paramount. In all of it's incarnation, Star Trek has portrayed the idea of a multi-species interstellar alliance.
Star Trek - Paramount. See Television.