logicalincrements.com Good as a starting point to find what kind of PC you want to build.
reddit.com/r/buildapcsales Aggregates best and latests deals on pc parts. You can also filter it, e.g.: latest SSD deals. This was probably the site that I most used for research, the comments sections were also very useful. Use the META filter for coupons and special deals.
pcpartpicker.com Use it to store your combination of parts and find possible incompatibilities between them. Given a piece, it also shows the prices from multiple stores and even the price history. These prices are sometimes out-of-sync or don’t take coupons in consideration, so sometimes you can get better deals.
camelcamelcamel.com Create e-mail alarms when a product reaches a desired price on Amazon.com. I have been using this site for everything related to Amazon.com with success for years.
iftt.com Create advanced notifications such “if there is a new post on /r/buildapcsales about SSD send me an email”. For that you can use an IF “New post from search” THEN “Send me an email” recipe with search as “subreddit:buildapcsales SSD”. You can replace SSD for anything you want.