In reply to Landy_Dom:
Deuter Speedlite - comes in a variety of sizes (mine is 20 litres, I think that there's a 10 litre one as well). No gear loops, but weighs almost nothing and (on the 20 model) you can use the side-straps to surround the whole thing and squish it to nothing! It also fits between the back mesh panel and main back board of my 'normal' rucksack for the walk-in.
That said, though not so much of a concern for climbing, it has no waterproofing at all (and indeed my version (the old style) has drain holes in the bottom!), and if you want to keep something dry in the rain, it'll need to be in a drybag!
I use it for day-walking/skiing/climbing - anything where I don't need the size or weight of a more substantial pack.