To each his/her own on mods and what you want to do. Here are the common ones.
Axle back, these are basically changing out the muffler hence axle back. The most popular I would say is MRT V2.0. You will never get the V8 sound from a V6 but these sound pretty awesome.
Cat back, this describes a system that starts behind the catalytic converters to the back of the car.
The only other option to our LFX engine in this department is high flow catalytic converters, or HFC's for short and as of now the only one I know making them is Solo Performance. This is the first section from the head to the first joint. Solo claims approximately 15 RWHP (rear wheel horsepower)
Cold Air Intake (CAI) is usually the most common under the hood replacement and there are many options out there. CAI brand is probably the most popular. I have Injen CAI. Injen claims 15 RWHP over stock. This replaces your stock air box and filter.
Ported Throttle Body, (TB) is another common replacement. I don't have a picture of this. This controls the airflow into the engine. Vmax is the main company that does machine work on these and what they do is smooth out the inside of the opening for better air flow into the motor. It is has a slight performance gain of 9 RWHP over stock and I noticed that my car has better mid-throttle response.
Oil Catch Can (CC) is another popular mod for under the hood. This is put in line on the crankcase ventilation system to stop the oil from entering into the engine. RX and Elite are the top ones. Elite pictured below.
These are the basic mods. The sky, or your check book, is the limit to what can be done. Here is a link to member car journals, http://www.camaro5.com/forums/forumdisplay.php?f=50
these are threads of cars that members own and have modded. Start a thread of your own and check out what others have done for ideas. The mods are endless. Feel free to check mine out as there are just a few other basic mods that I have done that is not listed here.