Cyclists who needlessly hold up traffic, typically lycra clad road bikers riding two abreast. I mean such a great way to attract sympathy hey? Share the road and such.
Actually, this is usually done for safety and not to be obnoxious. Bikes have every right to the FULL lane, so whether there are two cyclists in the lane or one, it doesn't really matter.
The reason they always seem to be clad in lycra is because people who put in a lot of miles tend to wear lycra and also know how to ride safely in traffic.