the most important resource you would not expect is.......

[IbnKhaldun]

Yeah, I remember when I realized how valuable vast expansive jungles can be. If you plan on using the State Property civic, they can be huge production cities*. If they have a river running through them, they can be the ideal cottage / commerce location (especially if you are Financial). Now with the National Park wonder... my goodness the possibilities are endless.
The irony is that the AI doesn't have the foresight to anticipate the late game like human players do. They avoid the jungles like (cough cough) plague, when even early (once you get Iron Working) they have huge potential.

* Another CivFanatic posted a fantastic article about using workshops to make grasslands into production wonderlands. Wish I could remember who wrote it and thank them again.

 

[SickCycle]

jungles are great if you have the time to cut them all, it takes forever..

i've been surrounded by jungles before and dropped a bunch of jungle cities immediately and it really can set you back awhile, i prefer to wait until i can afford to spit out ten workers just to clear some jungle that still takes fifty plus turns lol

 

[KMadCandy]

ghandi's UU is soooooooo fabulous for clearing jungle. walk on and start clearing right away so that you don't lose a turn for each one that's coming on the tile to work (as long as it's not a hill). it adds up!

even militarily they're powerful as an army corps of engineers, walking thru the jungle making roads in one turn (if you have enough to stack for game speed) so that your non-woodsman or whichever promotion that i call Tarzan) troops can walk on thru and head to the frontlines faster . i <3 those guys like nobody's business.

 

[mravioli]

the jungle is only good when cleared right? otherwise an attacking civ would also get a 50% def bonus