I have recently been asked to make a pledge to take actions in the future and I would like to have a legally binding and enforceable document to bind me to this pledge. Is there a means under U.S. common law to make a legal document that is binding upon a single party, but is enforceable?
The law does not recognize a contract with yourself.
The simple way out of this is to enter a contract with someone else. In general, a contract requires valuable consideration on both sides: you are providing a promise, they are providing? How about they will give you $1 when you do the thing - this is valuable consideration and would create a legally enforceable contract. You can also enter into a deed with someone which only requires consideration on one side but deeds are more complex than contracts and have some additional formalities that can be easily stuffed up. The dollar contract is the way to go. You can even gift your friend the dollar without affecting the contract.
This contract will be enforceable: it doesn't mean it will be enforced. If you fail to honour your pledge you have broken the contract, however, there are no legal consequences unless you friend sues you - this may not be something that the are actually willing to do.
You cannot be legally obligated to perform an act (or refrain from performing), unless required by law, or as part of a contract (the "common law" approach). One of the fundamental elements of a contract is that there must be consideration, an "exchange of promises". You can be held to do something, if someone gives you something of objective value in exchange. Otherwise, you don't have a contract, just a promise, which is not enforceable.
It depends upon the context and the reason, and it also begs the question of who would enforce it.
Some of the most common kinds of unilateral legal structures are single owner entities, declarations establishing condominiums and other covenant controlled communities, conservation easements, and trusts. Sometimes, the idea is that you establish legal rights with the idea that those rights will someday be enforced either by a transferee of an interest you create, or by a third-party fiduciary or protector whom you appoint irrevocably to enforce the terms of the legal arrangement. The fact that you have been asked to make a pledge certainly suggests that there might really be a counter-party even if you don't perceive it that way.
Certainly, not all binding arrangements require mutual consideration. For example, guarantees of other people's obligations rarely do, and neither do trusts.