Hex Nuts: We don't even need to discuss the DIY potential of this one, right? I mean, that bracelet that started it all.
Corner Braces and L Brackets: Make great necklaces, like this past project, and (wink, wink) perhaps a new DIY coming up.
Compression Sleeves: I love how this are available in a range of sizes (teeny tiny to upwards of an inch) and look like cool beads. The highlight of this popular Thanks, I Made it DIY hardware bracelet project.
Copper couplings: These parts, from the plumbing section, are perfect for adding a faux-rose gold touch to necklace. (Easiest DIY necklace ever using these here)
S hooks: These great connectors for wrapped rope bracelets, and there are usually tons of pretty colors of nylon rope at the hardware store.
Wing nut: Adds fun flair to a bracelet or necklace, like this project from Martha Stewart.
Washers: I've talked about Annie Alber's before, but her classic necklace (tutorial here from Chic Steals) using these disks is a hardware jewelry classic.
Really, the possibilities for creating fun jewelry and accessories with supplies from the hardware stores are endless, I can't even begin to scratch the surface. You can start by checking out past TIMI projects, and happy making!