Good advice in this thread so far. Here's my 2 cents:
ALK: is a no-no... at least if you ever think for a microsecond you'll be on an autocross course in the future. It also adds *very* noticeable NVH. I know, I've got an ALK with the *comfort* bushings in my garage (shameless plug: if you do want one, I'll sell it for like $50, it was on my car only long enough for me to find out it puts me in Modified
). An alternative are the Whiteline autocross legal offset bushings. Same polyurethane material as the ALK, nearly the same geometry benefits, no offset bracket = SM legal. If reuses your factory brackets, which makes the install a bit more involved, but it's cheaper, and works great (it what's on my car).
Steering rack bushings: awesome mod... kind of a PITA to install if you've never done it before, but really crisps up the steering, especially on a well tracked car that's beat the OEM rubber bushings to hell.
Rear subframe lockers: don't bother unless it's a dedicated track car. The handing gain vs. NVH is bad if you drive on the street... they put every bit of noise from the rear end into the back seats.
Trailing arms: the only real benefit are the better bushings... for the cost, there are *tons* of other mods out there. I've done nearly everything there is to do on my track car, and I have the stock trailing arms.
Lateral Links: Like Dean said, they rock... but again, they're one of those things that 99% of people could just get some camber bolts and be just as happy. Unless you're okay with lots more noise, and really need the adjustability to get wide rubber to squeeze under the rear-end... probably not worth the money.
Endlinks: I like the spherical bearings... much crisper turn in. And really the added noise comes from the swaybar bushings, not the endlinks. The only thing to think about is: endlinks are an intentional point of failure. If you beef them up, then when something breaks when you hit that pothole, it's not a $40 endlink, it's a $200 swaybar and mount, or a $400 lateral link, or something... they're great, but going for a massive dogbone of billet aluminum isn't necessarily the best thing for an otherwise stock car.