It's not online, but if you want to download something that's really small just to try out, you can try
www.theprodukkt.com. It's a functional 3D FPS that's only 96kB! I remember reading something about it in a past CGW, but never saw anything about it here. Sure, the sounds are crappy, but it actually looks fairly decent for a 96kB file. It takes a long time to load in the beginning though, so I wonder if it's actually secretly downloading more stuff in the meantime, or just reeeeeally well compressed.