The app that Oisin
mentions is awesome. You can use it to mark leads with a success status that you had in advance and add new leads that you meet at the event. A nice feature is that you do not have to be connect to Wi-Fi to use the app (as we all know event wif-fi can be spotty).
Tracking ROI can get messy if you do not have refined marketing operations. For simplicity, you will want to be integrated with a CRM, and make sure that you add a cost to all of your event programs.
If you are using Salesforce, you should sync your Event Programs with Salesforce Campaigns and make sure your sales team is adding leads/contacts to opportunities. You will be able to get some valuable ROI metrics in both applications. In Marketo, you can add opportunity information to your lead reports. Here is a helpful article:
https://community.marketo.com/MarketoArticle?id=kA050000000KypQCAS