The Cakewalk synthesizers book is well worth the price for the opcode listings alone. There are dozens of <effect> types - just about anything you can think of - and not all types are supported in every synth. With a knowledge of the basics, though, it's possible to create your own <effect> with combined opcodes:
Bigass Bandpass.
As for online examples, check the Instruments forum history. .sfz guru Chad has been posting a lot of .sfz examples (and samples) for years now. He also hosts some .sfz collections at PatchArena (I think I have some crazy FlexLFO stuff there myself). We had some end up in the Universal 120 expansion pack, too. There is good information out there to reverse-engineer, but it's scattered. Rene posted some choice nuggets online in the past as well.
garritan is using sfz formats now and they just got gobbled up by make music so there might be some developments there.
It'll be interesting to see that play out. It's a very well-thought out format that hasn't been fully
supported pushed like it should by the "powers-that-be".