For this job or in general?
For this job, you'll need a ratchet, a set of metric sockets and a variety of different length extensions (a long flexible one will help big time for the driver's side plug near the firewall) and some universal joints. Needle-nose pliers and/or a long, thin telescoping magnetic wand help if you happen to lose a socket in the engine. A spark plug socket is a nice piece to have too, or you could use a piece of rubber tubing/hose that has the same inner diameter as the back/porcelin/white part of the plugs so you can gently put the new plugs in and successfully take the old ones out. A tube of anti-sieze and dielectric grease are also good. That's pretty much it for this job.
Ideally, you should have a complete set of standard and metric sockets (regular, deep, and 3/4 length) in 1/4 in., 1/2 in., and 3/4 in. drives in both 6 point (to prevent rounding bolts) and 12 point (for the stubborn rusted bolts) and ratchets to go along with them, different adapters, a set of standard and metric wrenches - the ratcheting type are very nice, although sometimes it's just good to have regular ones, a variety of regular and Phillips screwdrivers, a few hammers, cheater bars, breaker bar, torque wrench, a variety of different kinds of pliers, a floor jack, jack stands, a dremel tool w/ a variety of bits, a drill with a variety of bits, trouble light, safety goggles, gloves, and a few other things that I forgot. On top of that, you'll need whatever specialty tools you'll end up buying along the way - I had to pick up a T-70 Torx bit, locking 3" 1/2 in. extension, an 02 sensor socket, a set of flexible extensions, an oil filter strap... the list really goes on, but that stuff should give you a pretty decent start. There's not really a single "complete" set of tools, but if you have one of every size socket from pretty small up to pretty big, that should pretty much encompass most situations you'll come across. You'll probably always end up running out for some things here and there, but if, for example, you don't have a 10 or 12 mm socket... that's something you shouldn't start the job without. Know what I mean?
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)