Here's a common sight in Austin: mustachioed hipster meets cowboy-booted New Yorker for the first time, but they're both staring down at their phones. Some of South by Southwest Interactive's breakout startups have many attendees heads down, tapping away while crossing 6th Street or standing around in groups in the Hilton lobby.

What are they doing? If not texting their rat pack with GroupMe (more on that later), they're hashing.

Hashable, the app for iPhone and Android, has been an instant success at the Austin geekfest, largely because it helps the guy wearing the Disqus T-shirt remember the dude he met at the Driskill. It's a perfect app for conferences where the volume of people met never corresponds to space left in your pockets. Leave your business cards at home -- this is how to trade details with Twitter, Foursquare and contact info that downloads directly to your phone.

Here's how it works: You download the iPhone or Android app (BlackBerry is coming soon) and fill out your contact info. Then you link to Twitter and Foursquare and step out into the SXSW fray. When you meet someone you could hire or pitch to later, you open the app, pick a hashtag like #justmet #beers or #with and then enter the email address or Twitter handle for your new friend. Once you've made the connection in the app, you can post that connection to Twitter or download their details to your phone, if they're still worthy of a slot in your address book the next day.

The best part is that you can track how social circles overlap -- you can designate your inner circle (translation: people that you actually care about, or whose circles you want to get closer to) and then watch who they connect with on the app. While it sounds annoying, it just goes to show how everyone in the industry you want to meet has probably met a friend or will share a beer with someone you know at a conference like this. It's like LinkedIn, except mobile and location-based because you can tag a place to each connection.

Say your old bud from your last gig just sidled up next to the head of social media for Coke at Stubb's, and they exchanged info through Hashable. You see the connection pop up on your phone. Later that night, you persuade your buddy to introduce you since your shop's been on the prowl for a soda client and he's already got one. This app draws all the lines between all the players to show that, really, there are probably only seven connections between you and Cindy Gallop. And probably less. Here, I also must say the app's been a little buggy for me -- it's slow and stalls and I'm not sure how two people ended up in my inner circle -- but I can't help but wonder if over-stressed wifi and faulty 3G may at least be somewhat responsible.

Hashable also wins for smart startup marketing at SXSW. While it's been hard to see the six-month startups for the big brands like PepsiMax and CNN that are dominating the show with their big tents and bands, Hashable is out in full force. Maybe it's the evangelists who will pitch you to death when you're trying to interview its CEO-founder and startup vet, Michael Yavonditte, a VIP party at the Driskill or Hashable T-shirt-clad folks at every party, but the New York startup has managed to rise above. One such T-shirt told me at the Foursquare party this is largely because the startup team is well-seasoned; many are on their second or third company.

(GroupMe too, with the help of BrewPR, has also pulled off startup marketing done right with its grilled-cheese-and-beer outpost a stone's throw from the convention center. While it's tiny in the shadow of the Sobe tent, it's centrally located, yummy and draws an interesting mix of investors, other startup guys and journalists.)

Hashable is still too early for a business model -- the startup launched in private beta last June and is now public. But, down the road, Mr. Yavonditte points out that advertisers could find business folk and road warriors when they're sitting down to meals with other professionals, all within the app. Two of the most popular tags are #lunch and #dinner. "They're like search queries," he said. "It's a powerful way to segment white-collar professionals."