Headache

Headache is a massive subject to try and cover. One website nicely splits them into three main groups - so I'll go over headache in each of these groups...

Common
    Migraine Headache
    Tension Headache
    Cluster Headache

Killers
    Meningitis/encephalitis
    SAH / sentinel bleed
    Intracranial Hemorrhage (ICH) (subdura/epidural)
    Acute obstructive hydrocephalus (and blocked shunts)
    Space occupying lesions
    CVA
    CO Poisoning
    Basilar artery dissection
    Preeclampsia
    Cerebral Venous Thrombosis (pregnancy/post-partum)
    Brain abscess

Maimers
    Temporal Arteritis (>50yrs & ESR)
    Idiopathic Intracranial Hypertension (pseudotumor cerebri)
    Acute glaucoma
    Acute sinusitis

References
http://www.doctors.net.uk/ecme/wfrmNewIntro.aspx?moduleid=1559
 http://www.doctors.net.uk/ecme/wfrmNewIntro.aspx?moduleid=1553
http://learning.bmj.com/learning/module-intro/tricyclic-antidepressants-headaches.html?moduleId=10017236&searchTerm=%E2%80%9Cheadache%E2%80%9D&page=1&locale=en_GB
http://jnnp.bmj.com/content/72/suppl_2/ii33.full
http://embasic.org/2011/08/27/headache/
http://wikem.org/wiki/Headache
http://emeddoc.org/?p=481
http://flippedemclassroom.wordpress.com/category/headache/
http://lifeinthefastlane.com/education/symptoms/headache/
http://secure.collemergencymed.ac.uk/code/document.asp?ID=5074
http://guidance.nice.org.uk/CG150
http://www.sign.ac.uk/guidelines/fulltext/107/