i'll add another vote for the holy trainer - it fits well, low profile and comfortable for long term wear. Although, i drilled several holes in the cage for better hygiene.

Next would be the spiked silicone devices - these are surprisingly comfortable device and have a ball splitter. The spikes are rubbery, so not overly intense, but enough to wake you up should you get an erection....these are available rather cheap through various overseas/china stores and are listed by many names.