zendesk. They have a pricing scheme for start-ups: http://www.zendesk.com/product/features/startups/

I'll second the recommendation for Zendesk. Their agent support portal is professional, highly customizable, and they are responsive and friendly. Their help desk portal is less mature, but quite a good value. We used it for a while with just one or two agents and loved it!

My only experience is using Zendesk, and while they're great for large teams, their pricing tiers can be frustratingly orthogonal to startup growth rates. You might want to use them for the long haul, starting immediately if your budget can handle it, or transitioning later. If you're using SalesForce for your sales teams, there are support modules there that can handle basic ticketing. It all depends on what you expect your support demands to be in the future.

I've built community and support from scratch at seesmic, uservoice, and launchrock. And, we worked through this same question at my current company, primeloop.com.

Desk, Zendesk, Uservoice, etc. are all great support platforms (and there are hundreds more). But I would skip all of those when starting off, and go straight to http://intercom.io since you can handle support through there, but you also get a full CRM and drip-messaging right out of the gate.

I'm more than happy to talk through your setup if you'd like to hop on a call?

I’d recommend Intercom as it is a perfect platform to start and then scale both, customer success & support teams without a need to develop custom features.

Asynchronous chats and automated messages that if emoloyed properly can help reduce support debt by sending out tailored recommendations, onboarding or announcement messages.

Wonderful tool for both, proactive and reactive customer support.