Here is a complete list of pre-order bonuses by retailer:
Microsoft Store:
Acordes de la Muerte Melee Weapon: Give me a "K!" Give me an "I!" Give me a "Guitar-with-melting-candles-on-it melee weapon that can bash in the skulls of my vilified enemies!" What's that spell? Errr...
The Hangover Melee Weapon: Show how much you like to party with this melee weapon made out of beer cans and an electrified pitchfork. One thing led to the other, right?
GameStop:
Wannabe OD Outfit: Be the talk of the town in this gorgeous and bold homage to your mutant enemies. Then kill them in it.
Head-Banger Gun: Get ahead of the game with this enhanced version of "The Dude". It does more damage and substitutes bowling balls with mutant heads.
Best Buy:
Fizzco Bot Outfit: Your 9-to-5 not making you feel enough like a corporate robot? Well, slap on this Fizzco Bot outfit and feel the joy of automated monotony!
AK-FOFF: An enhanced version of the AK-FU that features a graffiti'd exterior and inflicts more damage on enemies unfortunate enough to stroll by.
Koozie: Get an OverCharge can koozie!
Wal-mart:
S'more Action Melee Weapon: A campfire massacre isn't complete without your trusty marshmallow on a stick melee weapon to impale hordes of mutants with. 5-second rule!
Day One Edition content (also mentioned above, thanks Dormin), available I imagine through all retailers:
Nothin’ but the Hits gun: An overcharged version of the High Fidelity gun that launches limited edition, priceless, multi-colored vinyls that do increased damage.
It’s Me! Fizzie! outfit: Fizzie is more popular than you because he has a great marketing budget. So, why not dress up as him and get in on some of that fame?
Hardcore! Hammer: You ponder buying the Day One Edition of Sunset Overdrive. A spot check reveals that if you do, you can equip a mighty hammer made from a bat and spiked dumb-bells. You buy the Day One Edition of Sunset Overdrive.