This is a site I've taken for granted for so long, that it almost didn't occur to me to post about it. Open Source CMS
is a site that is dedicated to providing test environments of as many open source content management solutions as it possibly can. It can help immensely when you're going through that process of trying to decide which CMS is right for the website you're building - you can actually try the back-end of almost any CMS. The site gives you administrator privileges so you can really explore the features of the system you're testing. The only way they're able to do this is by over-writing each installation with a vanilla install once per hour but an hour is more than enough time to get a sense of whether a given CMS (and this includes blog engines) is right for you.