One of my favorites is a combination of two Logic plugins: Echo (in the delay list) and Tremolo (set to autopan, of course). Open Echo first on a bus and send your dry signal to the bus. Set echo as desired but with the effect set to 100% wet. It will play back in mono. Put Tremolo on the bus after Echo. Move the time dial to the left and select the desired tempo-sync setting. You'll know what you want when you hear it. You can then tweak the curve of the autopan by going with smoother panning or hard panning. Have fun. This setup sounds better than many paid plugins!