The two online patent search resources that I utilize the most to retrieve patent documents and to research patents are freepatentsonline.com and the uspto.gov website.
The uspto.gov  website provides two different databases. One for issued patents and one for published patent applications. Some published patents applications mature into a patent but some do not. Hence, the published patent application database and issued patent database have redundant data set. The bigger problem is that they are not coextensive. Hence, depending on the type of search you are conducting, you have to search both the databases. This is inconvenient and multiplies the work load by two.
Freepatentsonline.com  has developed its own proprietary database which is close to but not exactly the same as the databases at the USPTO website. Non-USPTO databases will not be identical to the Patent Office databases because of the manual filtering done by PTO personnel of the raw data. There are differences but I believe that they are generally minor. The benefit of freepatentsonline.com is that you can search both the published patent application database and the patent database at the same time eliminating redundant search results.
Other online patent search websites:
Getthepatent.com  is convenient because you can enter in a list of patents in bulk and retrieve the PDFs of the patent documents faster than freepatentsonline.com which you would have to enter in one by one. By the way, if you are retrieving single patents, freepatentsonline.com has a good interface.
Cobaltip.com  (developed by the same company as freepatentsonline.com) provides do-it-yourself patent searching but is a pay to use site. It has some unique features such as filtering instead of keyword searching. You can also obtain information on particular companies and gain insight as to what types of inventions they are seeking patent protection for.
Archpatent.com  which is similar to cobaltip.com is a free website for now. In my opinion archpatent.com has fewer options than cobaltip.com but may be sufficient for your needs. Both cobaltip.com and archpatent.com are worth investigating.
I invite you to contact me with your patent questions at (949) 433-0900 or James@OCPatentLawyer.com. Please feel free to forward this article to your friends. As an Orange County Patent Attorney, I serve Orange County, Irvine, Los Angeles, San Diego and surrounding cities.