A personalized letterhead is a great investment for big or even small businesses. Even personal mailings with letterheads that express who you are will make a positive impression to the recipient.
There are various tools available online so you can create one for yourself or your business. You may also buy a ready-made one from companies that specialize in online letterhead printing. Just be careful with whose company you'll put your trust to.
Simple, elegant, girly or whatever letterhead you choose, keep in mind that you are to give a good impression to your recipients. Avoid over decorated ones. Just use something that serves its purpose perfectly. :)