There's an app called SummitSync which has the swipe functionality so you can connect with people before the event, you sign in with your LinkedIn account.
There's Whova where you can scan in people's business cards when you talk to them and convert them into contacts. (CardMunch is a similar one)
We've used something as a company (before I started) where everybody at the event got a little device and you can put the devices together and it swaps your info. Can't remember what it was called but at the time I was using NFC tags for another event and it inspired me to think about putting NFC tags on the name badges and then programming them to the details of each person. Then when you scan someone's name tag with your phone you can save their details to your contacts. Obviously time consuming but extremely cheap!