For ravish night life, Kuta Beach has no equal on Bali. A 7 kilometer strip stretching north of the original village, packed with bars, nightclubs, massage parlours and whatever the thirsty backpacker needs for a grand night out - it's not pretty, but it's definitively happening.
Another cluster of nightlife can be found near Seminyak, which has laid-back, slick and very expensive bar-restaurant-nightclubs like Ku dé Ta and Hu'u, as well as the Bali insitution that is Double Six. The latter attracts huge beachfront dance crowds from 2AM onwards and frequently plays host to world name DJs. Seminyak also is the home of Bali's busiest gay scene a bit inland along Jalan Dhyana Pura.
For those intending to sample some nightlife, a good first stop is The Beat magazine (http://www.beatmag.com/). This twice monthly publication contains thorough listings of bar and club events as well as reviews.