This question has been asked and answered many times here -- it almost ought to be a FAQ. If you want to use RHEL without paying, you can request an evaluation copy, which I think will get updates for 30 days. After that, though, no more uodates, CentOS and Scientific Linux are, for most practical purposes, identical to RHEL, having been rebuilt from the source RPMs with the Red Hat branding removed. If you're just looking to play around with something RHEL-like, these are good options.
The main benefits of buying RHEL are the support and (if needed) access to RH engineers. It's not really designed for hobbyist usage.