I absolutely swear by L'Oreal Double Extension. Not the 'Beauty Tubes' one, just the regular Double Extension. I have the stumpiest lashes known to man (and am insaaaanely jealous of DH and DD because they have gorgeous, thick, loooong lashes!!) so I *need* a good lengthening one and so far this is the best I've found - and it's quite reasonably priced, too.
It is slightly volumising as well, but the best thing is that it almost doubles the length of my lashes so it's just what I need to get that lovely fringe-y look (most mascaras just make my eyeliner look darker, kwim, there are still no lashes there!!).

It's one of those 2-step mascaras with the white base to add fibres to your lashes (which lengthen them) and then the black mascara. The base is super-easy and non-smudgy to apply, the mascara itself is a tad sticky and can smudge badly if you don't give it a bit of drying time, but overall it's pretty fabulous - my only gripe is that the brush is a straight one (I prefer the curved brushes that get those teensy little lashes right in the corners of your eyes), but that's no big deal. Again, the benefits outweigh the cons so I recommend it!!