American designation (with European equivalents)

0
0Z4 - Full-Wave Gas Rectifier

1 Volt heater/filament tubes
1L6 - Pentagrid converter

2 Volt heater/filament tubes
2B7 - Twin-diode remote-cutoff pentode

5 Volt heater/filament tubes
300B - 40 Watt directly heated triode
5Y3
5751 - low voltage low-noise avionics tube

6 Volt heater/filament tubes
6AQ5 - (EL90)
6AU6A - (EF94)
6BQ5 - (EL84)
6C19
6CA7 - (EL34)
6CL6 - Power pentode
6DA6 - (EF89)
6DJ8 - (ECC88)
6J5
6L6 - (EL37)
6N3P
6SK7 - Remote-cutoff pentode
6SN7 - Medium-mu twin triode
6V6 - Beam power tube (see also: 5V6 and 12V6)

12 Volt heater/filament tubes
12AT7 - High-mu twin triode (ECC81)
12AU7 - Medium-mu twin triode (ECC82)
12AV6 - Twin diode/High-mu triode (see also: 6AV6)
12AX7 - High-mu twin triode (ECC83)
12BA6 - Remote cutoff pentode (See also: 6BA6)
12BE6 - Pentagrid converter (See also: 6BE6)
12DT6 - Sharp cutoff pentode

25 Volt heater/filament tubes
25L6

50 Volt heater/filament tubes
50B5 - Beam power tube
50C5 - Identical to 50B5 except for biasing arrangement (HL92)
50L6 - Beam power tube (see also 25L6)
50HK6 - Power pentode

Field emitter vacuum tubes

In the early years of the 21st century there has been renewed interest in vacuum tubes, this time in the form of integrated circuits. The most common design uses a cold cathode field emitter, with electrons emitted from a number of sharp nano-scale tips formed on the surface of a metal cathode.
Their advantages include greatly enhanced robustness combined with the ability to provide high power outputs at low power consumptions. Operating on the same principles as traditional tubes, prototype device cathodes have been constructed with emitter tips formed using nanotubes, and by etching electrodes as hinged flaps (similar to the technology used to create the microscopic mirrors used in Digital Light Processing) that are stood upright by a magnetic field.
Such integrated microtubes may find application in microwave devices including mobile phones, for Bluetooth and Wi-Fi transmission, in radar and for satellite communication. Presently they are being studied for possible application to flat-panel display construction.

Information from Wikipedia.