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Atomic Robo is a must get. In fact while you're there just buy all six issues of it. Or wait for the trade. In any case it's a fantastic comic from the writer of 8-bit Theatre. Tiny Titans is very fun too even though it's aimed at kids. It's one of my monthly buy books. The All-Star Superman has already achieved a legendary status among fans, so if that's more your taste, hit that.
And yes, of course it's free sample day! It's to get people into the comic shops to check out what's going on.
In that vein, if you've got 50 cents to spare, I recommend picking up DC Universe #0, which is sort of a preview/teaser of major storylines that DC comics is going to do in the next two years like Final Crisis, Batman: RIP, Wonder Woman: Whom the Gods Have Forsaken, Green Lantern: Blackest Night, etc. It's a full-sized issue and not a bad deal for 2 quarters.