The Star Wars/Serenity one is a fascinating concept: it's two versions of the same story. One features Han and Chewie and the other features Mal and the crew, but they both have essentially the same premise and arc, and both are written by Zack Whedon. (There's even a line of dialogue in common: both stories have someone offer to buy the ship; both Mal and Han respond with "Not for sale. Not now, not ever.") I think it's really quite a neat idea, taking two different licensed SF universes and showing how the same premise plays out in each.
The Serenity one is better. Partly it's because of the dialogue: Whedon, as you'd expect, knows the characters' voices, and it feels like an episode of the show. (Well, a very short one. But still.) The Star Wars story, rather by necessity, is mostly just Han talking, and isn't very interesting for it.
But partly it's because of the art. Fabio Moon puts in strong work on Serenity, and the colors are sharp too; the Star Wars story has decent enough pencils but it's got that digital-inking thing going that I don't care much for, and the colors are drab and brown.
Anyway. It's free. I personally wouldn't go to the hassle of signing up for a free Dark Horse digital comics account just to get it, but YMMV.
Some other publishers have put their freebies up online, too -- I'll try and get some links up later on.
