Great mod based off the BF2 engine. It's basically a large series of tweaks that make the game infinitely more realistic. For example, a new game mode called Advance & Secure. Has a similar flag structure as BF2, however you must secure flags in order. Bullet and gun physics and all the weapons have been worked over to make them act more like the real thing. The M249 is actually useful and you can't fire an M203 across the map.
In general, it's a much smarter version of BF2. A much larger majority of PRMM players use VOIP and squad work is much more important. Also, the longer spawn times (30 seconds + 1 second per death) make you want to play much more intelligently. Some people, however, find it too slow paced and boring if you are expecting the frag fest that is BF2. Anyway, anyone interested should check it out: www.realitymod.com
There are only two 40 man servers that are usually full, can find me playing under the name Foxbody later in the evenings.