JE wrote:
The common 1N4007 seem to work for HF but what is the max. frequency they
can be used as varactors?

And how about zener diodes?

JE
I suspect any si diode will work to some extent as a varicap or zener.
Not all diodes will be stable as zeners, the 1N506x series will probably
work better as a zeners than the 1n400x series. BTW the 1n5061 will work
as an 850v zener! Also the 1n4007 makes a good poor man's PIN diode for
switching rf.