1. I saw something very funny.

2. I saw something very fun.

(Are both expressions correct? What is the difference between 'funny' and 'fun'? Thanks a lot.)

The first one is correct. The second is okay, but in very informal speech (almost slang).

Take a look at the definitions of the two words:
http://dictionary.reference.com/browse/funny
http://dictionary.reference.com/browse/fun

The standard English use of the word "fun" is primarily as a predicate adjective (after a linking verb, modifying the subject), or as a direct object.
That ride on the ferris wheel was fun!
We had fun on the trip to the aquarium.

You can see from the two definitions that their meanings are close, but not identical.

Funny is an adjective modifying something. Fun is a noun.. Sentence #1 is correct.

I like GuruBlue's answer better!! Even though the dictionary says we can use "fun" in those different ways, those non-noun uses make me cringe!!

Both expressions are correct grammatically, but there is a slight difference in meaning between "funny" and "fun."

1. "I saw something very funny" means that you found something amusing or humorous. It refers to something that makes you laugh or brings a sense of amusement.

2. "I saw something very fun" means that you found something enjoyable or entertaining. It refers to an activity or an experience that brings you pleasure or excitement.

So, the difference is that "funny" refers to something that makes you laugh or brings amusement, while "fun" refers to something that brings you pleasure or enjoyment.