I use a TC Electronic D-Two for this. My preferred way of micing a room is two mics onstage - shotguns are my favorite but I've made use of Oktava MC012s, Earthworks SR77, even SM57 in a pinch. Large diaphram mics are good (Especially if they have a hyper function). This gives me crowd response, just be careful your screamers and off-key singers aren't right in the line of sight if you're using shotguns. Usually, most bigger shows that use in-ear monitors already have a set of guns or LD's on the sides of the stage feeding the crowd noise in the monitors, so it's not hard (if you're doing a pro job) to convince the band's crew to run you a line through the snake back to FOH for your mix, since it's split into the monitor snake already...
Then for room tone I put a pair of cards or hypers ORTF back by the board. You want something that'll capture a nice warm ambience of the room, and cut out some of the crowd. Since the onstage mics are more focused towards crowd response, you don't want the room mics to overload the crowd noise of the room. Usually for pro gigs I'll put a limiter with a slow attack time and fast release, with a sidechain/key of the board feed.. when the board feed gets hot (ie, the band plays) the room mics will fade down slowly a few DB to clean up the mix. I don't do this for the onstage mics as, like I said, they're more focused and don't have as much "crap" in them as the ones back at FOH...
Then the board feed comes in, delayed by the D-Two -- You need to be as close as you can be, and you need to constantly monitor it throughout the show. As the temperature and humidity increase in the room, you'll experience a change in delay times. You need to stay on the delay times or else you'll experience comb filtering (weird phasing). The calculation for this is pretty simple:
let's say S=the speed of the sound coming at you in air and D is the degrees farenheit in the room (a combination thermometer/hygrometer is great for this, I carry one in my pro rig for this exact situation). Let's say the room is 70 degrees.
S=(1052+1.10d) ft/s
S=1052+(1.10x70)
S=1052+77
S=1129 feet/second
So say your crowd mics are at FOH, and let's say that's 40 feet from the stage and the mains. The room sound will be coming at you 1129 feet per second. Since there's 1000 milliseconds per second, divide 1129 by 1000, that'll give you right at 1.13 feet per milisecond at that temperature... that means that the sound will reach your mics in right at 45 miliseconds. That's what your delay needs to be. You'll have to redo this calculation around halfway through the show and adjust your delay a small bit accordingly. Also, as the humidity level goes up (and if you're using a HQ mixer to mix down the 6 tracks) you might want to either roll up the bass or roll off the high end, because the high end will become more pronounced with higher humidity.
If everything's mixed down properly, and no phase problems come up, the mix should be awesome...