ST = Stainless
DAO = Double Action Only (You probably knew that.
)
DAK = Double Action Kellermann. There are two pulls with this trigger design. The first is a long pull which is fairly light (~6 lbs, I think), the second is a short but heavier pull (~8 lbs, I think). The shorter, heavier pull is to prevent short-stroking and is achieved when the trigger is not reset fully. Kellermann is the name of the guy who designed it. These will be hard to find since it seems most (if not all) SIG DAKs are going to government/LEO contracts.
Nitron = Finish that looks blued, but is more durable.
P210 = a full-sized 9mm, single-action with single column mags. Regarded as one of the most accurate stock pistols out there (very expensive, too
).
P220 = a full-sized .45 ACP with single column mags. Was made in other chamberings and can be found if you look hard enough, such as the 9mm P220 with European style mag catch.
P225 = a compact 9mm with single column mags.
P226 = a full-sized 9mm, .357 SIG, or .40 S&W with double column mags. .357 SIG and .40 S&W P226s can swap between the two with barrel changes (same for other SIGs that come in both .357 SIG and .40 S&W).
P228 = a compact 9mm with double column mags.
P229 = same dimensions as P228, but also can be had in .357 SIG and .40 S&W, along with 9mm. Differentiated from the P228 by the half-height slide serrations.
P230 and P232 = sub-compact .380 ACPs with single column mags (I think a .32 ACP version used to be produced, too).
P239 = sub-compact 9mm, .357 SIG, and .40 S&W with single column mags.
P245 = compact .45 ACP with single column mags.
SIG Pro 2009 = full-sized 9mm with polymer frame and double column mags.
SIG Pro 2340 = full-sized .357 SIG and .40 S&W with polymer frame and double column mags.
Note that compact SIGs are usually a bit larger/bulkier than compacts of other brands. All are traditional DA/SA (except P210) unless noted as DAO or DAK.
SIG Pros are generally regarded by SIG enthusiasts as being very underrated and a very good deal because of their price.
That's just from the top of my head and in the most common chamberings, so some info may be missing or incorrect.
I only have a P228, but I love it. If I had more money I would have many more SIGs in various models.