I can also suggest Tender for basic usage. One supports for $9 (and more if you add supporters, but a small startup shouldn't need more than that.
Let's you get some structure in there and can be scaled upwards at a decent price.
Far from perfect, but functional for sure.
I considered this for my work, but didn't end up using it, purely due to how our existing processes didn't really fit with the workflow of the product.
However, my experience with it is that its a high quality and very versatile product, used by some big names in the web scene. For example, this is tenderapp: http://support.cheddargetter.com/
I'm not going to list off the features but I suggest you try it out, they have a ~~free edition~~ trial edition.