Here are list of websites that are useful.
Most up to date HEX card database with lots of additional features such as syncing deck builder with actual game data, calculating value of your collection, and listing missing card etc.
This site parses HEX official data and lists results of tournament deck contents and their results. What it means is that players can look to see what decks are tier 1 in construct etc.
This site now hosts weekly in game tournament, which is free to participate. The site also provides great strategy articles. Now banned Majest deck was originally introduced on this site.
The site also has great articles but rate of update is slower than the Fiveshards. The site also uniquely provides option to buy in game card for real US$.
The site dedicated for PvE Campaign mode information and most comprehensive one out there.
This is composed by one of well known community member, NicoSharp, on the HEX forum. It lists top tier decks for Arena grinding i.e. fast, reliable decks.
Auction House Related
This site uses official HEX data and gives up to date auction house trading information. You can see which cards are gaining in value or losing in value. Other information such as gold to platinum ratio is also useful.
Official HEX site. The site now has almost a daily update on weekdays. Major news update however is usually every Friday afternoon in US time.
Another official HEX site. This site gets major update a day before HEX TCG site i.e. Thursday in US time.