I need to drill a couple of holes in the bottom of a kitchen cabinet to fit it with IKEA Akurum legs. I thought of using a 1/2" spade drill bit but its' narrow end - the tip of the spear so to speak - protrudes too far and would drill through the 3/4" board before the 1/2" diameter cylinder could reach 1/2" depth.
Carefully chop the posts on the Akrum down by 1/4" using a band saw or a dremel. Now drill a 3/8ths deep 1/2" hole using either a twist drill or a spade drill. Dip the Akrum in a bit of epoxy or wood glue and install it. This will provide the same centering and strength, but will minimize the likelihood of drilling through the cabinet, or (worst-case) of making the hole too short and having the weight of the cabinet fall on the post rather than on the rectangular structure around it.
As a practical concern, given the comments that you made, it doesn't seem that if you drill the hole all the way through anything bad will happen, except that you'll have some surface imperfections on the floor of the cabinet.
Forstner bits (as mentioned by @keshlam) have much less of a protruding point, and would do an adequate job of drilling a 1/2" hole without poking though the remaining 1/4"
If you pick them up cheaply (bucket-o-bits at the flea market or yard sale) you can also grind off most of the point on a spade bit - start the hole with the normal "long-point" version, then change to one with the point severely shortened.
In the event that you want a truly flat-bottomed hole (which does not seem essential here), and if you happen to have the tools (buying them for the sake of making holes for cabinet legs is not economically sensible) a 1/2" plunge (or "end-cutting") router bit and and a plunge router will get you a totally flat-bottomed 1/2" hole 1/2" deep (if you set the plunge depth to 1/2", and hold or clamp the thing firmly in place so you son't cut a slot.)
Use a standard 1/2 inch twist bit. These bits like to jump around a bit until they bite the wood. To to make it easier, start with a good depression for the bit to start in. For smaller bits, I usually just use a punch (if you don't have one, a nail will work) to make a small dent in the wood. For larger, I like to use a smaller bit to get a hole started. For a half inch, I would probably go with a 3/16 bit.
As for depth, if your drill doesn't have a depth stop (sometimes they have a mount in the body or handle for a rod to be attached) you can always use a piece of tape wrapped around the drill bit at the correct depth.