I used to do this to my girls' jeans after school let out. Great way to boost the summer wardrobe for next to nothing! Transform an old pair of jeans into adorable summer shorts using bias tape. Instructions on the site.
25 Brilliant Clothing Items You Didn't Know You Could Buy - Secret Bra Stash: This is the perfect, secret way to stash your valuables (cash, emergency credit card, etc.) while traveling, or even just a drunk night out with the girls.
Tutorial for sewing your own knickers (panties). I love this as I prefer full coverage knickers but they are usually only available in granny styles. Now I can make them in stretch lace and all kinds of pretty fabrics.