I have been using LaTeX for quite some time but never on a daily basis. As I've used it to write many different documents I came across many useful packages but I have trouble keeping track with all of them.
Do you have any suggestions on how to keep a record of the LaTeX packages that you have already used along with a brief description of their functionality? It would be nice if the solution included using a single file or a web service.
May I suggest organizing your preamble by grouping related packages and commenting liberally? This response to Best practices on organizing your preamble can be extended to keep a dummy master preamble file where you can archive all the packages you have ever used, in neat categories, with brief descriptions of their functionality. Of course, you wouldn't want to use this file as your actual master preamble, because it will change all the time, but you can pick and choose the packages you need from it and use them in your documents.
Also, texdoc <package name> on the command line will pull up the package's documentation, if you need more help on using a specific package. See the texdoc quickguide for more information.
fancyhdr. For other packages that deal with headers, tex.stackexchange.com/q/1247/3731 offers a few suggestions. Regarding your additional question, tex.stackexchange.com/a/18550/3731 also gives a good suggestion for the general case. If you have something specific in mind, you should post a new question with a MWE. It helps others find a lot easier.