Ten Things About Guidelines for Unagented Submissions
1. Wholesome romance, mystery and historical writers might want to check out Avalon Books guidelines.
2. Baen Books publish only science fiction and fantasy (I'm guessing alternate history falls under the fantasy flagship, since I sold them an AH story), and their manuscript guidelines are here.
3. How to Prepare a Proposal for Cambridge University Press.
4. Carolina Academic Press welcomes submissions for "titles in law and other academic subjects" and has an online submission consideration form.
5. Dorchester Publishing (Leisure, LoveSpell) have their guidelines here, along with a comprehensive list of what they're currently acquiring.
6. Harlequin has a bunch of imprints and guidelines for them here, two of their newest imprints Nocturne and Kimani Press, the latter of which is incorporating their Arabesque, Sepia and New Spirit imprints.
7. Guidelines for Kensington (Zebra, Pinnacle) Books as well as a list of editor names and what each handle.
8. Want to write a Star Trek Novel? Check out Simon & Schuster's ST novel guidelines.
9. The Tor Official Guidelines; not very informative, but they do accept unagented submissions and have a fairly decent response time. Submission guidelines for their paranormal line are here.
10. Another inspirational publisher, Zondervan Books, is looking for academic, reference, and ministry resource proposals and accepts electronic submissions.