The Gaslamp District is the place to go if nightlife is what you're after in San Diego. Many bars and clubs to choose from.  Dress code, long lines, selective doormen and high cover charges typically apply.

The younger surf/sorority/fraternity crowd (21-30), or the casual crowd usually heads to Pacific Beach. Some also head to Ocean Beach, although Ocean Beach is historically known for the bohemian crowd and harleys.

Gay nightlife is centered around the Hillcrest area, north of downtown proper, and spreads somewhat into the adjacent communities of University Heights and North Park.

From dive bars to upscale clubs, there are plenty of nightlife options: 

On Broadway, located at 5th and Broadway, in downtown San Diego, is an upscale club housed in an old bank. It still contains the original vault, round steel door and all. Inside, where thousands of dollars were once stored, you can now dance to Latin 
music and play pool. The whole club had an aura of being very classy and upscale - the sleek booths and chairs, the elaborate wall sconces, the dramatic light shows, the beautiful college aged clientel. Be prepared to pay for it though; admission is $20 per person, and drinks are very expensive - a rum and coke is $6.50 (Captain Morgan $7.50) and shots run $10.50 and up. During the early evening, you can dine on sushi upstairs at the Zen Cafe. After 9 PM the tables are pushed away to make room for dancing. Downstairs, along with the vault room, are two additional rooms with bars and dance floors. The VIP Room is guarded by another set of bouncers and velvet ropes. Inside, there are private booths good for talking and a dance floor in the back. From Mission Beach, USA cab will pick you up promptly, although the 15 minute ride to downtown San Diego is pricey - $20 each way. (Please check prices - as always they can change.)

If you're looking for a more laid back club with a decent dance floor, The Bitter End, located at 770 5th Avenue, fits the bill. The upstairs holds a long, shiny bar with plenty of stools, a pool table, and following the carpeted red stairs, a VIP room, not open during the week. Downstairs there is an underground dance floor with air conditioning. It may remind you of a friend's basement. The dj plays Top 40 and some 80's music. Drinks are cheaper than On Broadway but shots are still expensive (two Sour Patch Kids cost $15). With friendly bartenders and a laid-back atmosphere, The Bitter End is a great place to chill upstairs or dance the night away downstairs.
A few blocks away, Pacific Beach Bar and Grill, at 860 Garnet Avenue,  is a perfect place to play darts and pool. On the weekend, the place gets packed. Bands play regularly on the stage and there are indoor and outdoor seating areas serving food. Drinks are average price - and the cheapest  found in Pacific Beach - $4.50 for a Captain and Coke, and $2 for a Guinness.

Sunshine Company, located at 5028 Newport Ave. in Ocean Beach is similar to the more popular clubs in Pacific Beach, but the crowd is much more laid back. It is a two level club with an open courtyard so smokers don't have to go outside and can still be with their friends. Drinks are moderately priced. Connected to the bar is a taco shop so you can order rolled tacos (or your personal favorites) while you drink. The upstairs area has tables and overlooks the interior of the bar. It can get fairly packed on the weekends after 11, but if you are looking for the best place to meet people in OB, Sunshine Company is the place.