Might a "raindance" be something other than an American Indian tribal ritual? One wonders.
One of the longest running and largest rave events in the UK, Raindance events are generally well organized and attract a devoted and energetic crowd. Unfortunately the Drome (at London Bridge) was closed in 2001-February by the Southwark Council, but reopened in 2001-May allowing Raindance to return from exile (Camden Palace). Some tragic events in 2001 compelled the closure of the Drome again (see tom_f's node), and it is not clear what the future will hold for Raindance and its organisers.
Raindance events are sanctioned and draw large crowds (Drome capacity is 4,500, though there is invariably more demand than space). The DJ's are generally excellent (and there are many of them-- spinning simultaneously in various rooms). Drink plenty of water, be mature with the MDMA, and enjoy the plur. Raindance is legendary.