For small domestic projects, or situations where only a few holes are needed, a handheld post hole digger is your best bet. Smaller and more portable than their larger, tractor attachment counterparts, handheld post hole diggers are usually operated by a single person, though 2-person models do exist.
If you're in need of a portable post hole digger but need something with a bit more stability and power, a cantilever style machine might be the right choice. Cantilever machines use the engine as a counterweight (and base), and have a pair of wheels to make transporting and moving around the work site easy. These post hole diggers are ideal for small commercial projects that are too big for a handheld machine, but too small for a tractor or larger machine.
When you do need to dig post holes on a larger scale, then a post hole digger tractor implement is the way to go. As it is attached to your tractor, this digger will generally be more powerful than a handheld model, and will be more efficient. This also makes them a better choice for tough, hard to dig earth where that extra bit of power comes in really handy.
If you run a smaller tractor, don't worry. You don't need to have a large 200+HP tractor to operate a post hole digger, there are plenty of brands out there with post hole diggers for compact tractors and machines with less HP. The power your tractor can put out will affect performance, but this usually isn't a problem and in most cases you will never notice any difference.
There's more to digging holes than the power of the engine; there are also different augers available that create different digging effects. Most brands will have a variety of augers available to suit different situations (like clay vs dirt, or muddy/sticky material vs loose dirt).
Whether it's for your backyard, farm or a fence for a commercial client, post hole diggers make installing and replacing fence posts a breeze. Do your research or contact your local dealer to find out about the right post hole digger for you.