Add me to the dummie group. I can do 2D collision, 3D bounding spheres, but when it comes to BSP, Portal, I''m lost. I''d like to post a note about this because YES, I get tons of requests on this topic. If someone is willing to write a very easy to understand, commented collision demo that works (no sliding through walls, etc), using the code from my site as base code, I will pay $50 for the code. I know it''s not much, but it''s out of my pocket, and the could would help everyone out! I''ll convert it into a tutorial once I''ve had a chance to go through the code, and figure it out

Quake3 didnt use sphere collision detection, it uses bounding box hull checks..sphere detection is pretty ugly when it comes to walking around on ledges and over uneven geometery - check system shock 2 or thief - horrible physics feel there (good games though)

If you jump about in Quake1-3 and Half Life you can pull pixel perfect accurate jumps and ''bunnyhop'' about the place from jump to jump - with sphere collision games u can often become ''stuck'' in geometery and have to wiggle your way out by randomly spamming the jump button and moving in all directions - my 2d engine so far uses raycasting to check the movement and collision of objects (and correct them) and it feels pretty fluid at the moment

It''s not often a boundingsphere, or even a convex hull test is usefull for anything but an early rejection. Once you get a collision for the spheres, you still need to get down on a per-polygon level. You''d also want to have the exact feature-set of the collision, and the penetration-depth. And this is where the hard part comes in.
Gino van der Bergen wrote a very good book on the subject.