I've personally tested the following LED lamps.   You could use a MR16 Halogen lamp.   They only cost 1-2 EUR, but to me the high current draw and short battery life are not worth the money saved over a LED lamp.    

 This 5W 12V MR16 LED (Cool white 6500° K) lamp with 10° beam angle is the best lamp I've found for when it is real dark.    The lamp draws 600 milliamps and is ideal for the handle bars or the helmet.  According to the manufacturer the lamp has 1,100 Lux @1 meter!  The lamp is not sealed and is not water tight.    This lamp will probably blind car drivers and I do not recommend it for city riding where there is already some existing street lighting.  For summer time use the lamp should get some ventilation to help keep it cool.  

For city riding, this lamp could be "toned" down for street use using 8 AA rechargeable batteries instead of 10.   There are standard battery packs for 8AA cells available.  Using 8 AA cells the lamp draws about 500 milliamps.   Typical rechargeable batteries are 1.2-1.3 volts.   8 AA cells would provide about 10 volts instead of 12.5 volts

This 3W 12V MR16 LED lamp with 25° beam angle is an option for city riding.  This lamp draws about 350 milliamps and should improve the battery life over the 5W lamp by 42%.    It is a good intermediate solution for riding on the streets and areas where it is dark.     The lamp can be purchased from Conrad for 19.95 EUR.   The lamp is not sealed and is not water tight.

This 1W 12 W MR16 LED lamp with 15° beam angle is an option for city riding, where you only want to be seen.  You cannot see much with it.   The lamp draws just 70 milliamps and would go for quite a long time without charging of the battery pack.    The lamp can be purchased from Conrad for 9.95 EUR.   The individual LED's are exposed and set in a base of white silicon rubber.  

Halogen sealed Lamps:  Not recommended

1.   Current draw is too high for inexpensive Ni Mh batteries

2.  The Tubelight housing would insulate the lamp too much and get too hot.