Foam in the oil cap

I've noticed that inside the oil cap (engine 3VZ-E) sediments some kind of dark yellow foam, I haven't seen it before. What could it be? Is it something serious or normal?

There is a good chance your head gasket has developed a leak and coolant is getting into your oil. Have someone you trust check it out right away. It is serious.

What should I look for to be sure?

A milky color in the oil is one tell-tale sign. Another type of leak in the head gasket would make alot of bubbles come out of your coolant overflow tank (from the piston compression).

A white smokey exhaust is another.

(also, bad head gaskets are common with your engine)

Unless someone else disagrees with me, I think the discoloration on your oil filler cap is enough evidence and you should get it fixed immediately to make sure there is no engine damage due to oil contamination. You might be fortunate and have caught it early enough that you haven't seen any additional contamination.

Thanks.

Is there a possibility that I have crack in the head not just in the gasket?

Yes, but head gasket is a somewhat common problem with that engine. 90% chance it's a bad head gasket. Once the head is off, the problem will be obvious.