Grey tulle dress, $79.98 (~£49) on sale at White House Black Market. I think this is adorable, it reminds me of the Chloe S/S11 ballerina dresses.
Tulle ballet skirt (£310) by Wheels & Doll Baby, available from asos. Way over the average person's budget for a skirt, but I could see this being paired with a white halterneck corset to make a fab bridal outfit.
Maddison dress (£150) by Coast. Pricey, but for a formal occasion I think it might be worth the investment, the finish on the halterneck looks incredible (and would be difficult to recreate without serious sewing skills).
A quick look on Etsy revealed a couple of gorgeous vintage bargains:
1950s Prom Dress (£19.20), culturedecay on Etsy. This is dying to be shortened to cocktail length.
Halter party prom dress (I think it's a tulle underskirt and organza outer) (£33.28) by thevintagemistress. Would NOT suit my colouring, but I think it would look stunning on someone with warm, golden skin (i.e. Springs)
If prom dresses are a bit too girly girly, I think this tulle slip from 23stien would be amenable to dressing down. I really love this, it has an internal unitard!
And just to drool over, the most expensive (and loveliest) tulle dress on net-a-porter, by Victoria Beckham (I hate to say it, but as a designer, she has very good taste):
A staggering £6,850.