I've been to some of these, but not all. I just did a bunch of research for my trip on the nightlife (so I'd know where to go or not go), and I thought it's pretty good info... (and it's a culmination of many, many different websites)...
Clubs on the Strip:
V Bar (@ the Venetian) - No cover - It's one of the newest clubs in Vegas, in one of the nicest hotels, so it attracts lots of celebrities. Inside, it's very pretty with simple Mod furniture and dim lighting. The music is mostly jazz and "quiet" techno. Drink prices average at about $10 (Martinis are $14) and the bartenders come up with new drinks every single week so be sure to ask what the latest concoction is.
Jack’s Velvet Lounge (@ the Venetian) - No cover - A small club with a great atmosphere, where everything is white except for the exposed wooden beams on the walls. There's also a nice outdoor patio where you can look out at The Mirage across the street. Lots of locals hang out here, but the crowd is pretty mixed. No dance floor, but they do pump trance and house music in through speakers.
C2K - (@ the Venetian) - $5 for girls - A four leveled club, with lots of couches, chairs and plenty of bars.
Club Utopia - (next to MGM Grand) - opens @ 10:30 - Cover ranges from Free to $20, depending on the night. It's touted as "THE underground club in Las Vegas" and the whole club inside is completely white. Top DJs from all over the world spin the latest in dance music. Also, it's been said that sometimes lines of club-goers suddenly appear at this club at 8 in the morning!
Drai's (@ Barbary Coast) - $10 cover - An after hours club (Opens at 3 AM). Before I went on my trip, I talked to a bartender in Houston that lived in Vegas for several months that said this was THE club to go to. While I was in Vegas, I met a muscle-head guy at Baby's that lived in Vegas who also told me that Drai's was the best club in Vegas. I have never actually been there, but based on these two guy's glowing reviews, checking it out couldn't hurt.
Baby's - (@ Hard Rock) - Always no cover for girls, $20 for guys - It's called the "secret" nightclub, but you can usually find it by the line (if not, find the bathrooms, it's near there). The crowd is young and single. The club consists of 2 separate rooms and a VIP area where you can check out the dance floor action, an oxygen bar and a lots of pools of water. Can't get into the VIP room? In the main room, head past the dance floor and go "behind" the DJ booth. It's somewhat more private back here, and not as crowded as the rest of the club. Drinks were pricey: $4 for beer, $7.50 for a drink, and $8 for a shot of tequila.
Circle Bar (@ Hard Rock) - no cover - In the center of the lobby of the Hard Rock Hotel. A great place to begin any night. It's always packed with young singles, and the drink prices are pretty reasonable for Vegas. Lots of people hang out here before heading into Baby's... Celebrities are known to hang out here too.
Rumjungle (@ Mandalay Bay) - $20 Cover (even if you're staying there!!) - Even though they play a nice mix of hip-hop and techno, it's too crowded inside (after you wait in the hour or so line) to dance or get a (expensive) drink. Yuck.
Red Square (@ Mandalay Bay) - no cover - This place rocks. It's a martini lounge located next door to Rumjungle. Although it plays no music, it does attract a lot of the young, fun crowd tired of waiting in line next door. Their specialty is (duh) martinis and your drink will always be cold, since you'll keep it on the long , thick strip of ice running the length of the bar. For something good and different, try the rootbeer float martini - Yum!
House of Blues (@ Mandalay Bay) (doors open 11 for dancing) - Cover is $15 - On Fridays, it's '80s night. Saturday is more modern music. Earlier in the nights there are sometimes concerts or other events. For a schedule, see their website.
China Club/The Dragon (@ Mandalay Bay) - Opens @ 3AM - Located around the corner from RumJungle, lots of celebrities hang out here. Good luck getting in - the door guy is very discriminating.
Light - (@ the Bellagio) - Opened in December 2001, it's the newest club in Vegas. Good luck getting in: if you want to sit at one of the 37 tables surrounding the bar and dance floor, you better reserve it advance and get ready to shell out the dough: Instead of a cover, each table is expected to buy bottles with bottle prices start at around $250 a pop, with a 1 bottle minimum for a table of 4. Don't want a table? The general public is allowed in (if you meet the dress code, of course), but only if you're a girl or possibly a couple. Be sure and stop by P. Diddy's table and say hello.
Drink! - (close to Aladdin) $5-10 cover - Supposedly the best place to hear hip-hop music in Vegas. With eight different-themed bars (some only lit by candlles) inside the same club, there's sure to be something for everyone. There's lots of drink choices: from the 75 micro-brewed and imported beers at the beer bar or 20 types of vodka at the vodka bar. Celebrities have been seen here.
Clubs off the Strip:
Voodoo Lounge (53rd floor of the Rio) - $5 cover, large specialty drinks ($7-20) - AWESOME view of the strip, nice laid back atmosphere, attentive waitstaff. There's a jazzy band that plays inside and a small dance floor surrounded by tables. Outside - more tables & the great view.
Club Rio - (@ the Rio) - $5 cover - Just like any big club in Any City, USA: Top 40, hip-hop, and dance music and cheap drinks. Also known as a "meat market."
Ghost Bar (55th floor of The Palms) - Take the super-fast lobby elevator to this indoor-outdoor lounge and sky deck. With it's 12-foot floor-to-ceiling windows, you'll have an awesome view of Vegas. Head outside to get a 360-degree view of the skyline. There's a VIP lounge, a dance floor to dance to the electronica music, all underneat a ghost-shaped ceiling that changes colors with the music.
Rain (@ The Palms) - $20 cover - A multi-level club and concert venue, located on the Casino level. Millions were obviously spent on the gorgeous decor - from the elevated bamboo dance floor to the computer-programmed "river" of water, fog, and pyrotechnics. Millions are also meant to be spent here: either in the private VIP rooms (you enter via a scanned thumbprint) that include personal bartenders, cabanas with mini-bars, and specialty furniture or perhaps in the private booths with water sofas or the skyboxes with private balconies and self controlled interior experience. On the back of the concert stage, there's a huge color-changing water wall, and in the front of the stage is a "curtain" of water pouring down.
The Beach - (365 Convention Center Drive) - No cover before 10 (No cover for girls all night) - Spring Break anyone? The crowd is casual, young, bikini-clad, and wild. Tiki hut decor, with everything you would expect from a club in Florida.
Glo - (1650 E. Tropicana Ave) - No cover till 1 AM (opens at 11) - A really new club that gets "good crowds on Fridays and Saturdays." Saturday night is ladies night and the music is Hip-Hop and R&B and there's a complementary champagne toast.