Homegirl,
While I'll leave the whole write up about gym etiquette to Dan, I will give some pointers here.....
1. Do not walk in front of someone (to put down weights, to get to a machine, etc.) when they are using the dumbells in front of a mirror. Nothing irks someone more than when you are in the middle of a heavy set and you're checking your form in the mirror and someone either walks in front of you or just stands there...
2. Wipe off the machines with a towel/paper towel after you are done using them if you have sweat on them. Obviously there are a lot of sweaty people in the gym but no one wants to workout in your sweat!
3. If you are not sure if someone is still using a machine, ask before you sit down and start using it. Sometimes people just go to the water fountain in between sets.... it's better to ask than to have someone come back from in between sets and you be on their machine.
4. The gym is not social hour! While you can talk and have a good time, remember people are trying workout. Do not go stand next to people that are trying to workout and laugh obnoxiously..... (FYI...I like to socialize at the gym but I also know not to stand real close to people and be obnoxiously loud!)

Ok, that's all I can think of for now.... Dan, I'll leave the rest to you (please add to your list about "stalking".... )