[Mahn]

Wikipedia is probably the most dangerous procrastination tool on the net. There's not limit there, the rabbit hole just goes deeper and deeper.

[Fernicia]

tvtropes should also be considered in this category.

[krylon]

Oh my, yes! I went there to look up one term, and before I knew it the entire afternoon had gone by.

OTOH, I learned a lot that afternoon. Watching TV programs has never been the same.