*** Find out how much ships costs, what modules you can fit, and what performance you can expect from your current/next ship. ***
Coriolis - I just wrapped my head around this site and found it really useful. Not only can you do outfitting and see your power output vs. power needs, it also allows you to fiddle with module's power priority settings! This way you can see whether you can get by with your current power plant or if you really must upgrade. It also has the ability to save ship loadouts and compare loadouts which is great! The only downside is that the module costs are listed in a separate section which sometimes makes it hard to find how much that next module upgrade is going to cost.
E:D Shipyard - a simpler version that still shows fitting options as well as jump range and power output vs. power needs. No ability to mess with module priority or save loadouts on the website although there is an "export" and "import" function, but you will have to save the data somewhere else, ie text file or Excel.
*** Need a new ship? Want an upgrade module but don't know where to find it? ****
INARA - signing up for the website allows you to set your current location on the CMDR'S LOG section and then going to the GALAXY section, you can search for ships (SHIPYARD) or modules (OUTFITTING) and it will show you the nearest known locations that may have what you are looking for. I say "may have" because I believe this is crowd-sourced information and may be wrong or out-of-date. So far, I've bought an Imperial Eagle/Courier/Clipper and have outfitted them with the help of INARA and just this evening, I've searched for A-rated modules and with INARA's help, I found what I was looking for on my first stop.
*** Want to trade but don't know where or how? ***
Elite OCR - dock at a station, take screenshots of the commodity list, run the screenshots through Elite OCR, import the data into your favorite program, and let it crunch the numbers for you! Basically, Elite OCR saves you from all the typing.
CMDR's Log - save your Elite OCR data as .csv files and import them into CMDR's Log and this program will crunch the numbers, show you which stations to trade at and what commodities to buy/sell, as well as the expected profits from the run.
Slopey's BPC Market Tool - similar to CMDR's Log but with the option to download crowd-sourced (online) data.
(Thanks MojoFlow!)E:D Market Connector (EDMC) - I am guessing this is similar to Elite OCR? I will be testing this shortly...
(Thanks MojoFlow!)Thrudd's Trading Tools aka elitetradingtool - an online version similar to CMDR's Log/Slopey's BPC, obviously with crowd-sourced data.
(Thanks Chucky!)* A note to the new traders * I am not a fan of crowd-sourced data and I only use them to gauge how profitable my trade routes are compared to what other players find out there. The reason I don't like trading in systems found in crowd-sourced data is simply because with the information being public, you can be sure that the best routes will attract more traders which can raise buy prices and lower sell prices which equals lower profit per ton. Another reason is pirates - with known trade routes, you can be sure there will be human players about who aren't engaged in trading.
My suggestion is to find your own trade routes and to do this, you simply fly to a few stations, grab the data, import it to CMDR's Log or Slopey's BPC, and find out what is profitable for you. The best routes are usually from Extraction/Refinery systems to High Tech systems. Depending on your ship size, you may consider outposts for small/medium ships but large ships will need stations.
Obviously, if you play mostly or exclusively in Solo mode, then human piracy won't be much of an issue and will make crowd-sourced information much more useful but again, there is always the problem of the data being out-of-date and you fly into a system to find the buy/sell price to be different and you end up making less profit than you expected.
These are the websites I have found that I use quite often; if anyone has other tools that they find very useful for new and experienced players alike, post 'em here and I'll add it to this first post.