There are many ways to find prior art (books, thesis, magazines, patents, etc etc).
When doing online prior art search, what tools and databases do you use?
In my corporate life I used paid services such as Micropatent and TotalPatent. Of the free patent search tools, I prefer The Lens over Google patents. The Lens provides more fine grained control and if you register (completely free) provides useful additional features such as saved searches, search history and collections. In addition, I find The Lens provides more accurate searches. Google Patents sometimes, inexplicably, doesn't find patents that The Lens does. I also think Espacenet is really useful for patent searching. There are other good options. I've heard decent things about Free Patents Online, but haven't used it much myself.
I do use Google Scholar for non-patent searching although I do think paid services are probably stronger. The Lens has some new capabilities in this area, but it is immature as yet. Other sites you might try out are Microsoft Academic, Science.gov, PubMed Central, and WordWideScience.org.
There is a lot of skill to doing prior art searching and professionals are really useful as resources. I've worked with professional patent librarians and they use techniques such as patent classification codes to make searches more accurate. If you are associated with an academic institution, you should really consider utilizing your school's library and librarians.
Google patents, google books, google, patbase and library catalogues sometimes.
I haven't seen any tools delivering (good/better) natural language results or anything like that (though many promise to).
I'd love to be proven wrong on this one as a search is just tedious work, but as of now I haven't seen anything beat a good old
search - understand what you're finding and why / why not what you were looking for - search more
approach with varying keywords.