Comixology have a number of free issues, although most of these are Share Your Universe branded issues. As with Marvel Unlimited, it updates with a new set of free issues every Tuesday.
Comixology also has Marvel's first issue of G.I. Joe from the 1980s, republished by IDW.
Some mid-2000s Marvel issues are available here, although some links are broken.
The 2007 Spider-Man FCBD story Swing Shift, which established the Brand New Day status quo, is available on Marvel Wiki.
The FCBD issues of Infinity (2013), Rocket Raccoon and Guardians of the Galaxy (2014) available on Comixology.
- The Avengers Prelude: Fury's Big Week #1
- Captain America: First Vengeance #1
- Share Your Universe: Captain America and Thor! #1
Fun for fans of the show. Otherwise? Nah.
The Point One story of Ms. Marvel presented incomplete in a lackluster preview not built for Infinite Comics. Eh, the art's nice.
A very good short story by Dan Slott from the All-New Marvel NOW! Point One anthology which works really well in this format.
Free with the code DDROAD (expires 10/31/2015) - thanks to ewokpelts on Bleeding Cool for this one.
Iron Man: Mark VII - An Avengers motion comic which acts as a prequel to the film, tying together threads from Iron Man 2 and brings forth Tony and Pepper's relationship.
The first issues of the Astonishing X-Men and Spider-Woman motion comics are available on Marvel's official YouTube channel.
Some PSA issues:
The infamous Spider-Man/Power Pack PSA from 1984 where Peter reveals he was sexually abused by a friend...
Avengers: Heroes Welcome on Comixology and Marvel Unlimited, a surprisingly good vignette by Brian Michael Bendis focusing on Nova. Out of all the PSA stories, it feels the most true to character and least heavy handed.
The Harley Davidson series which should only be read for its ridiculousness; essentially it's the modern black leather version of Marvel's Team America (this was a thing.)
Essentially what would happen if Tony Stark were not addicted to alcohol, but constantly worried over his 5 a day and at least six glasses of water.
Pepper Potts test drives an Audi and Iron Man fights Blizzard. It's kind of fun.
Some all ages content is available on the Marvel Kids website, but more is on the main site.
Mary Jane #1-4
Mary Jane: Homecoming #1 (#2-4 appear to be broken)
Spider-Man Loves Mary Jane #1
Franklin Richards: Happy Franksgiving! #1
Franklin Richards: Super Summer Spectacular #1
Marvel Age Spider-Man #1-20
Marvel Age Spider-Man Team-Up #2
Marvel Adventures The Avengers #1-2, #4-15, 17-19 7
Marvel Adventures Fantastic Four #0-31
Marvel Adventures Hulk #1-16
Marvel Adventures Iron Man #1-8
Marvel Adventures Spider-Man #1-4
Marvel Adventures Super Heroes (2010) #1, #2
Power Pack (2005) #1-4
Avengers and Power Pack Assemble! #1-4
Hulk/Power Pack #1-4
Iron Man/Power Pack #1-4
Spider-Man/Power Pack #1-4
Interestingly, I've found that issues from the prior incarnation of Marvel Unlimited can be accessed by right clicking the 'read sample' icon on the book you want to view, with the outdated viewer intact and everything. What I've found out of is that some previous free comics are available in full:
The Amazing Spider-Man #534, #535, #536 and #537 (Civil War)
Annihilation #4, #5, #6
Captain America #22, #23, #24
Captain America Comics #3, #4 and #6
Civil War #3, #4, #5, #6, #7
Deadpool (2008) #1
Origin #1, #2, #3, #4, #5 and #6
X-Men #157, #158, #159, #160, #161, #162, #163, #164 and #165 (Day of the Atom by Chuck Austen; #165 is a Christmas issue written by Claremont)