Your kegerator is probably currently set up for Sanke kegs. I don't think you want to use snake kegs for home brewing, but you may want to be able to use Sanke kegs or pin/ball lock kegs. I think there are adapters that will allow that, but when I bought a commercial kegerator, I knew I'd only be using it for home-brew, so I just chopped out the Sanke parts.
As BlackDuck mentioned, pin locks and ball locks are different sizes. Some people have kegerators that will fit an extra keg if they're all ball lock kegs. That's not an issue for me, since I only have a single tap tower (actually, I've got a dual tap tower, but haven't gotten around to hooking it up, yet; but even with the dual tap, I'll have room with pin locks).
Another difference is that ball lock kegs have a pressure relief valve in the lid. Most pin lock kegs don't. If you need to vent, you need to open the gas-in. There's actually a special tool for this, but you can also just use a screwdriver. One drawback to this is that if you overfill the keg, the gas-in dip tube may be partially submerged and will spew beer when you depress it to release the pressure (yes, I did learn this through experience and not through reading; and yes, it was messy). The lids are a standard size, and you can always replace them at a relatively small cost. After a little research, it looks like there are a couple of standard sizes. Some pin lock kegs ship with lids that have pressure relief valves, but that's unusual.
One thing that's nice about ball lock kegs is that it is impossible to accidentally connect the gas to the liquid line and vice versa. One has three pins and the other has two. With ball locks, although the posts are a slightly different size, it's possible to connect them incorrectly, and I've read that it can be difficult to disconnect them when this happens (no personal experience). It's probably unlikely that this will happen unless something happens like a keg running out while you're drinking, but I'm not sure when a keg would run out unless you're drinking.
Ball locks seem to be more popular, but pin locks seem to be less expensive (maybe because they're less popular).
I posted a link recently in the steals and deals section about a deal on a 4 pack of kegs, but that was a first-come-first-serve sale and it's over.
However, I just checked
Keg Connection and they've got a couple of sales going on. Their kegs are pressure tested and cleaned and also include a set of new o-rings (always a good thing to replace on kegs that previously held soda), unlike the ones at Northern Brewer. They have a
4 pack of ball locks for $189.95 and a
4 pack of pin locks for $119.95. They've also got
"Grade B" ball lock kegs for $44.95. That makes them slightly cheaper than the 4 pack ball locks but more expensive than the pin locks. Note that these have cosmetic damage, which may include damage to the handles that can make them harder to move around, especially inside the kegerator, unless you do some repair work. In my opinion, it's not worth the small savings.
I'm a cheapskate, so I'd probably buy the pin lock kegs. You could buy a
reconditioned lid for each keg and the pin lock 4 packs would still be less than the ball locks. The only reason I'd go with the ball locks is if I needed to fit more kegs in the fridge and the pin locks wouldn't fit. On that note, you may also want to check the dimensions to make sure your kegerator has enough height for the ball locks. If it's too short for ball locks, you don't really have a choice.
If you have the budget for it, buy two of the pin lock 4-packs. You can replace the lids at a later date if you decide it's necessary. But I think 8 kegs is kind of a sweet spot, and this is a great deal. To give you an idea of how great this deal is, I got to a point where I had too many kegs and sold some off for $45 apiece.
One thing to consider is that prices on used kegs seem to be rising pretty steeply. The first two kegs I bought had damage to the handles (probably similar to the Grade B kegs above listed for $44.95). They cost me $17 each. I think the "normal price" on used kegs back then (2011?) was about $35-40 ($35 for pin lock and $40 for ball lock). I haven't priced in a while, but I think it's more than double that now.
I don't have any affiliation with keg connection, other than having bought things from them in the past. When I bought from them, the had $7.99 flat shipping. They may still have that, but I haven't bought from them in a few years.
If a vendor doesn't state their shipping and doesn't have flat rate, make sure you check shipping before you buy. The first kegs I bought were $34, and the shipping was almost $30.
A few other things to consider is that you'll need some other things to keg.
You'll need connectors (either ball lock or pin lock). I'd advise getting quick connect connections so that you have the flexibility to use either type in the future (although almost everybody I know sticks to either one or the other).
You'll also want keg lube. If you're buying from one place with flat shipping, get a bunch. You'll use it all the time.
There are keg and line cleaning kits and I bought one, but I honestly just use an Oxiclean or BPW soak and run through the lines and never use the kit.
Consider Perlick or Intertap taps. They're less likely to clog and leak. Perlick taps are kind of the gold standard, but I recently bought an intertap and it seems to be pretty similar. If your current taps work for you, there's no reason to change, but if you travel a lot and end up with taps that don't pour, those two are worth considering (I think I got the best deals on taps from rite brew).
