Vaisala Lightning Explorer
Vaisala Lightning Explorer displays recent lightning activity across the entire continental U.S. The lightning data displayed is 20 minutes delayed and updated every 20 minutes. Get the latest map available by clicking "Refresh" under the map. The map shows a 2-hour time period with lightning data color coded in 20-minute increments.
Lightning Data from the U.S. National Lightning Detection Network
Vaisala Lightning Explorer uses lightning data from Vaisala's U.S. NLDN, the most reliable lightning detection system in the U.S. The U.S. NLDN constantly detects lightning discharges anywhere in the continental U.S. Each symbol on the map represents one recorded lightning event.
Go ahead and bookmark this page for a quick check before heading out to the lake or ball field. But know that the data is delayed by 20 minutes so it's useful, but by no means "real time". If you want fresher, more current data you'll have to subscribe to Vaisala, but they have a number of different options, some quite affordable (if you have a weather-sensitive business and lightning is a constant threat). The 20 minute delayed lightning information can be found here.