Technical Engine question...

[Wavesonics]

So I am a hobbyist engine programmer and am always interested in things like this that don't come out of the big production houses.

So I was wondering what type of technologies you guys used to solve the common problems.

For hidden surface removal what do you use? Oct Trees? BSP Trees and Potentially Visible Sets? Do you pre-calculate visibility? Do you do fulstrum culling?

Is the engine portal based? What are you using to cross-platform things like window creation, are you using SDL?
Is the engine based around a single thread?
Is the engine built around a client server model or strictly a single process?

It's a GREAT engine guys! I'd love to hear a little bit about what it's doing under the hood if you wouldn't mind sharing!