A leaking baseboard heater is something that needs to be repaired as soon as possible. A leak of this kind creates several problems in addition to the obvious lack of efficiency that will be present in your heating system. It can also damage your floors and can cause the growth of dangerous molds and mildews in the area of the leak.
A small leak should be fixed, but isn't a cause for immediate concern in regards to the heating system itself. Hot water heating systems have a connection to the main water supply from the home. So, the system retains the water it needs by drawing it from the main water supply. Be sure to clean up any leaks quickly however, and bleach the area if you suspect it has been damp for some time. This prevents the growth of mold.
Finding the exact source of the leak is important when deciding whether the problem is repairable, or if the unit should be replaced instead. Trace the path of the water back along the system to see if you can pinpoint the leak. Look for moisture directly on the radiator and look for cracks, pinholes or other signs of leaking. Sometimes you might trace the leak back to a valve or elbow, in which case the piece probably needs to be replaced by a professional.
A leak repair on a hot water radiator is a fairly involved process, although it can be done without professional help if the leak is small. The radiator needs to be drained first, and then cleaned very thoroughly with a substance that cuts all the grease away from the surface, such as acetone. Then you file the surface to create a place a the patch will adhere to. Finally, in the "Old House Journal", Dan Holohan recommends that you patch the leak with JB weld according to the directions on the package.
Some leaks signify damage significant enough that your baseboard heater needs to be replaced. Look for cracks or splits. Pinhole leaks can usually be repaired, but once the metal has started to split it is difficult to make it strong enough again to sustain the pressure load exerted on heaters. Another bad sign is significant rust. Even if you patch one leak in a rusty baseboard heater, it is likely to spring another in the near future and replacement is probably a better option.