50kV Voltmeter
Be very careful with this. It does work and is accurate enough for hobbiest work.
Basics of High Voltage Probe Design
Covers theory, tips, pratical advice, sources, parts, links to other sites, and information about HV probes uses and design.
High Voltage Probe
and Multimeter, Outokumpu Technology, Carpco Division
Measuring high voltages
A fairly accurate safe way to measure really high voltages.
Sphere gap breakdown voltage tables
How to measure that HV with some precision.
Vitrek
Manufactures hipot testers for both AC and DC measurements including a complete ground safety, fault locating, and measurement systems.