There are a lot of products out there that can do double or even triple duty if you know a few little tricks....
Here are some:
A brown eye pencil. This can be used as eyeliner, to fill in brows (brush through afterwards with an old clean mascara wand), a natural eyeshadow if you blend it well over the eyelid to the crease and also a lipliner - lightly trace the outline of your lips and blend with a cotton swab until it is barely there.
A pink, peach or red lipstick can double up as a cream blusher. Apply after foundation and smooth onto the apples of your cheeks for a healthy flush.
The foundation in the lid of your bottle. This tends to thicken up and so is perfect as a concealer, bonus - it's the same shade as your base!
Shimmer powder can be used to highlight cheeks, eyes and the cupids bow and if mixed with some moisturizer it becomes a great all over body shimmer.
Loose face Powder has a multitude of uses! The ones that work best for me are as a dry shampoo - dust on with a clean blusher brush, massage gently into the roots and then brush well. This soaks up oil at the roots and helps freshen. I also use loose powder under my bra when I'm wearing a strapless one, to help keep it in place and if you dust it onto lashes between coats of mascara it really helps thicken them!
Petroleum jelly is great for all sorts of beauty treats. Mix it with eyeshadow to give a glossy editorial look. On cheekbones for a sheen, lashes for a natural conditioned look and lips to keep them soft. You can also put a little down your shin bones to make legs look slinkier!