Nicest/biggest frame (ideally silver if budget allows) then HUGE collage of pics of all the diff things you two have done together; places been together (you get drift!) etc. Hugely personalised, gorgy to receive, and kinda symbolic of end of one chapter of her life and start of another. If in doubt re pressies, I ALWAYS buy either a photo frame; gorgy vase/s; or candles - all pretty fail-safe!
I'm always wary of buying something that I expect someone to have in their home - you have to be so careful you're not imposing your taste on them. If someone got me a massive collage of photos of us together it would go up when they came round and come down when they left - far too studenty for me, but then that's just me, you know your friend best.
Why not pick a piece of jewellery out together that you know she'll really cherish and think of you when she wears it? Something timeless and classic that she can keep for years to come.
A little keepsake of the invitation, confetti, couple of photos, menu, cd of the music played as she walked down the aisle etc and a comments book (can't think what they're called) that's got a little message inform all the guests at the wedding/reception?
Tickets to somewhere they'd love? A little romantic weekend away in the summer with champagne in their room?