AVG Free Antivirus.
Here's a snapshot in Process Explorer. While some of its processes are 32-bit, the important one -- avgrsa.exe, which is the resident shield that scans accessed files -- is a native 64-bit process. The proof's here:
Anyway, I wouldn't get too hung up on everything you run being 64-bit. It's often not necessary, and the performance difference is negligible for most applications.
(By the way, I own a registered version of AVG and I also didn't include all of the options during install, so what you see may not reflect exactly what you'd see in the process list for the free version of AVG.)