I've done a CCIE exam since (one of 4 or 5) which I think means my CCNA gets extended, but I haven't bothered to check properly.
The last Cisco exam I did was pretty clever, they have put quite a lot of effort in to trying to make it relavant and practical by 'simulating' IOS and real products. It's really like trying to set up the real thing.
The CCNA exam I did a few years back had a lot of useless stuff in it, there was to much focus on stuff people just didn't really use any more (Particularly IPX) but i'm told they have significantly revised it to bring it up to date since then.
You certainly can 'go in cold' for the Cisco exams. (And video conference certifications by Polycom). But you want to be really certain about the subject.
I went and did the CCNA exam having just spent 2 weeks reading the Cisco press exam primer. My wife did (and passed) the CCNA exam at the same time WITH OUT even reading the exam primer!.
But we both work with Cisco gear, she's working on it nearly every day. (Sitting on her desk at work are two cisco routers, one with a VIOP card, and 2 old cisco Access servers).
If you wanted to do the certification exam so that you could get a job in the field, doing the exams with out doing a course is going to be pretty hard, as you won't have any actual experience on the equipment.
Note the actual exams are generally NOT free, they can be pretty expensive on their own, and the courses generally have the exam as part of the package, so when considering the cost of the course, remember that a chunk of that course fee is the exam fee that you will have to pay anyway.