I really can't wait to play Steam games with my Mac friends, but they all own varying versions of new macs and mac books, but will they even be able to play something like L4D2? I don't really know much about macs or what their gaming capabilities are, so I was hoping someone could fill me in?

Most Macs are moderately powerful computers. You should be fine. I wish they were cheaper though...

Here is the specs for the current Mac Pro's. 
 
 

Quad-Core	8-Core
One 2.66GHz Quad-Core Intel Xeon "Nehalem" processor	Two 2.26GHz Quad-Core Intel Xeon "Nehalem" processors
3GB (three 1GB) memory	6GB (six 1GB) memory
640GB hard drive	640GB hard drive
18x double-layer SuperDrive	18x double-layer SuperDrive
NVIDIA GeForce GT 120 with 512MB	NVIDIA GeForce GT 120 with 512MB
Ships: Within 24hrs	Ships: Within 24hrs
Free Shipping	Free Shipping
$2,499.00	$3,299.00

Just for laughs here is the higher priced model all pimped out with the fastest processor and the most ram....  This sucker is a Multimedia designer's wet dream!!! 
 
 

• $10,049.00
• 
  
• $10,049.00

Mac Pro

Remove Save

• Ships: 4-6 business days
• Delivers: by
• 
  
• Part number: Z0G1

-

Edit size and color

Configuration

• Two 2.93GHz Quad-Core Intel Xeon
• 32GB (8x4GB)
• 640GB 7200-rpm Serial ATA 3Gb/s
• 4x NVIDIA GeForce GT 120 512MB

Games like Left 4 Dead and Team Fortress 2 can run well on even lower end PCs so it should be fine on most Macs.

$2,500? Thats just a bit ridiculous, just a bit.

Macs are overpriced to HELL. 
 
You can get a PC with 3 times the power as a Mac for the same price.

source games will run meh 
they run on my imac with a shitty ATI 2600 XT that doesn't even use PCI express

Mac guy here. I can run almost every game on my steam list on high. Especially Valve games, they run the best....
 

" Macs are overpriced to HELL.  You can get a PC with 3 times the power as a Mac for the same price. "

Don't know why you would use a Mac for gaming when you get a 4x stronger PC at the same price :/ . But anyway, Source games aren't very demanding now and older Macs or Macbooks should be able to play at medium-high settings, the newest iMacs should be able to play at high-highest settings. 

Would an imac with a Radeon HD 2600 PRO run games well?

a lot more than the average pc. 

" Would an imac with a Radeon HD 2600 PRO run games well? "

I got a low-end MacBook Pro that can run GTA IV acceptably while running windows. Seeing as how that game was coded, that's saying a lot. No problems with games like COD games and such. L4D2 shouldn't be a problem, especially since the game engine is like 5-6 years old now.