Common beta emitters
Strontium-90 is a commonly used beta emitter used in industrial sources. It is also been used as a thermal power source in radioisotope thermoelectric generator power packs. These use heat produced by radioactive decay of strontium-90 to generate heat, which can be converted to electricity using a thermocouple. Strontium-90 has a shorter half-life, produces less power, and requires more shielding than Plutonium-238, but is cheaper as it is a fission product and is present in a high concentration in nuclear waste and can be relatively easily chemically extracted. Strontium-90 based RTGs have been used to power remote lighthouses. 
Strontium-89 is a short lived beta emitter which has been used as a treatment for bone tumors, this is used in palliative care in terminal cancer cases. Both strontium-89 and strontium-90 are fission products.
Neutron activation products
Tritium is a low-energy beta emitter commonly used as a radiotracer in research and in traser self-powered lightings and some "glow-in-the-dark" paints. The half-life of tritium is 12.3 years. The electrons from beta emission from tritium are so low in energy that a Geiger counter can not be used to detect them. Note that one of the first-aid treatments for the intake of tritium (as tritiated water) in a human is to give the human plenty of water to drink. This means that the tritated water will be mixed with normal water and will therefore be flushed out by the body more rapidly.
Carbon-14 is also commonly used as a beta source in research, it is commonly used as a radiotracer in organic compounds. While the energy of the beta particles is higher than those of tritium they are still quite low in energy. For instance the walls of a glass bottle are able to absorb it. Carbon-14 is made by the np reaction of nitrogen-14 with neutrons. It is generated in the atmosphere by the action of cosmic rays on nitrogen. Also a large amount was generated by the neutrons from the air bursts during nuclear weapons testing conducted in the 20th century. The specific activity of atmospheric carbon increased as a result of the nuclear testing but due to the exchange of carbon between the air and other parts of the carbon cycle it has now returned to a very low value. For small amounts of carbon-14 one of the favoured disposal methods is to burn the waste in a medical incinerator, the idea is that by dispersing the radioactivity over a very wide area the threat to any one human is very small.
Phosphorus-32 is a short-lived high energy beta emitter, which is used in research in radiotracers. It can be used in DNA research. Phosphorus-32 can be made by the neutron irradiation (np reaction) of sulfur-32 or from Phosphorus-31 by neutron capture.
Nickel-63 is a special isotope of nickel that can be used as an energy source in Radioisotope Piezoelectric Generators. It has a half-life of 100.1 years. It can be created by irradiating Nickel-62 with neutrons in a nuclear reactor.