I was looking at the Costco website, and was surprised to find a really good deal on a gym membership. Costco is offering a two year membership to 24 Hour Fitness for $299. That works out to $12.50 a month. This includes access to most 24 Hour Fitness Clubs (except the highest level ones).
I've been a member of 24 Hour Fitness for 8 years in the Bay Area (and I though I had a good deal, paying about $16/month). The classes are good, and most of them have kids' clubs (hours can be slightly restrictive, in that the kids' club is closed mid-day and on Sundays) . My kids have a love/hate relationship with the kids' club, but it's an inexpensive place to leave them while you're on site ($2/visit per kid if you buy a 20 visit card, otherwise it's $3/visit/kid).
Most of the clubs have pools, a co-ed jacuzzi, steam room and sauna, as well as locker rooms. Of course there's a weight room and cardio equipment. The staff is friendly (though turnover can be high). And they're generally clean. The regular clubs aren't very fancy, but if you're only paying $12.50 a month, who cares?