When most people hear Ancestry.com, they automatically think subscription genealogy site. While it is primarily a subscription site, it's a little known fact that a number of databases on the site are available for free to the public.
The free databases cover a variety of categories from census to christenings to naturalizations, and everything in between. It's not just records from the United States either. The databases cover multiple locations around the globe including Poland, Prussia, Norway, France, Belgium, Canada, Ireland, Scotland, Lithuania and Australia, just to name a few.
To check out these free genealogy resources, go to Ancestry.com and look at the card catalog (found under the search button dropdown in the main toolbar). In the keyword box, put in the word free and then hit search. At the time of this post, it returned 449 results.
One thing to remember is that these free databases may or may not include the actual image of the document. Some will only search the database index for your ancestor's name.
You may be asked to register before you are can view the results of your search. Registration is free and also gains you access to the message boards where you can submit queries about your research interests.
A free trial may be offered. It's up to you whether you take it. If you plan to use the trial, I would suggest first making a list of what you want to research before starting the trial so you can make the most of your free access. If you don't plan to subscribe, make sure to mark on your calendar when the trial ends so you can cancel before you get charged.