I rarely post, but this is a topic that I've been doing a lot of research on lately. I currently working a web design studio of 5 people, and a few sub contractors. Our current software is Streamtime.
Streamtime
I hate it. We all hate it. We all do, but we're yet to find any other software that offers as many features as this software does. I think my greatest reason against it is how annoying the UI is to work with. We're all Mac based and it really doesn't tie into the beautiful UIs that we're used to working with. It's damn expensive too. At the moment, we're paying $50AU per user per month. The larger your business, the more expensive it gets. You have the option of buying it outright at the start, then paying for upgrades, but even then, you're up for an expensive purchase.
The only reason we're still on this software is that we don't have time to migrate to another system, and my boss wants all the extra reports/options, but for the designers/developers, there are so many annoyances that seem so simple that it's unbelievable that they haven't fixed them yet.
so1o
I gave this software a crack last week, and visually, it's amazing. Web based, which is very important (to me at least). Quite a bit more limited, and suitable to individuals rather than teams. They have a multi-user version on the way in the coming months, which I'm keen to see. At the moment, it doesn't quite have the right features/customisability required for me (as a freelancer) but I'd recommend all freelancers have a look at it.
Pancake
Web based software. It's a one-off $49 purchase which you then install it on your own PHP/MySQL server. I purchased this a couple of days ago and am in the process of setting it up. I've noticed a couple of minor issues/questions that I'm not too sure about, but it looks quite good. Two decisions led me to purchasing this app:
It's built using CodeIgniter, so at the end of the day, I can customise it to do anything that (I think) it lacks; and
Their developers seem quite open to suggestions on their forums and the pace at which new features are being added seems quite good. Anything this software doesn't do right now, I'm sure it will in the coming months.
At the end of the day, which software you choose really depends on the individual and their needs, so I think your best bet is to try as many options out as possible and you'll eventually find the one app that suits you best.