Blood tests are laboratory tests done on blood to gain an appreciation of disease states and the function of organs. Since blood flows throughout the body, acting as a medium for providing oxygen and other nutrients, and drawing waste products back to the excretory systems for disposal, the state of the bloodstream affects, or is affected by, many medical conditions. For these reasons, blood tests are the most commonly performed medical tests. Blood is obtained from a patient by venipuncture, except for tests such as Arterial blood gas.
Blood is useful, as it is a relatively non-invasive way to obtain cells, and extracellular fluid (plasma), from the body to check on its health. Although the term blood test is used, most routine tests (except for most hematology) are done on plasma or serum.
The list below includes both specific tests, and general techniques.
The CHEM-7 test is a battery of blood chemistry tests. The seven parts of a CHEM-7 include tests for:
Blood chemistry tests
Bicarbonate or co2 
Blood urea nitrogen (bun) 
Glucose . (This test can also be performed directly by diabetic patients by using a glucose meter.)
Large organic molecules
Protein electrophoresis (general technique -- not a specific test)
Western blot (general technique -- not a specific test)
Liver function tests
Serology (general technique -- not a specific test)
Wassermann test (for syphilis)
Cold agglutinins test 
Polymerase chain reaction (DNA). DNA testing is today possible with even very small quantities of blood: this is commonly used in forensic science, but is now also part of the diagnostic process of many disorders.
Northern blot (RNA)
CBC or Full blood count (or "complete blood count")
Hematocrit and MCV ("mean corpuscular volume")
Erythrocyte sedimentation rate (ESR)
Cross-matching-Determination of blood type for blood transfusion or transplants
Blood cultures are commonly taken if infection is suspected. Positive cultures and resulting sensitivity results are often useful in guiding medical treatment.