Do you think your genealogy research has some value to others besides you and your own family? Or perhaps there is no one else in your family interested in history, and you want your research to be passed on to someone who will appreciate it.
Whatever your reasons might be, you may be thinking about donating your family tree findings to a library or public archive. What better way to share what you have learned and possibly help others to find those missing links in their own family trees?
The best place to start looking for an appropriate archive would be your local library, regardless of what locations are found in your material. Your own library can help you locate other library branches closer to the areas found in your family tree.
Most local archives are really only interested in local reference information, though any genealogical society tied to the archive might have broader interests. Once you contact an archive, find out what their "range" is. If your family tree is spread out geographically, you may very well have to break your collection apart and donate chunks of it to different locations. Though regrettable from your point of view, it is understandable. A collection of small town history won't have the need to house photos or documents from 200 miles away.
Also be prepared to only donate your source documents, rather than your personal genealogical information. In other words, your own pedigree charts aren't likely going to be valuable to a historical archive. They will want old photos, newspaper clippings, maps, vital records, and other reference material. Genealogical information may be of interest, if enough people in your family tree lived in the area.
All archives are different, as are their needs and requirements. You may find that a small collection is only interested in certain photos, where a large historical society may be happy to take your entire body of research, personal pedigrees included. If you are not happy splitting up your material, try contacting a genealogical society, rather than a library archive.
To get a rough idea of what some archives will want, here are some online collections from archives around the country:
Before deciding where to donate your records, also check to see how the archives are stored. If the "archive" is a pile of cardboard boxes in the back storage room, you really should keep looking. A proper historical archive will have their collections properly stored and protected.
Remember, you are donating your material. That means you will be giving it up, so make sure you are finished with your own research and have copies of the items you want to have on hand. You will no longer have any rights to the documents and will only be able to access them through the archive as any other visitor.