In brief: One of the better modern MMOs, Elder Scrolls Online, is going free-to-play for the next week and change, and its various DLC packs are getting steep discounts. This means you'll have access to all of ESO's core premium features, such as crafting material storage and its Morrowind DLC, at no cost.
When ESO first came out, it wasn't exactly well-received. It had its core fans, but the multiplayer features were rather buggy and unfinished and many fans didn't find enough content variety to keep them hooked. Vast swathes of land around Tamriel weren't yet finished, preventing players from exploring them.
These days, ESO is much, much better than it was before. I've returned to it multiple times and enjoyed myself. I have a few complaints regarding the combat and writing (I think both are a bit dull) but, overall, it offers a massive world to explore with plenty of compelling quests, characters, and gameplay systems to uncover.
Normally, ESO would run you $20 for the base game. During this week-long free trial period (plus one extra day), that price has been slashed to $6. This nets you both the Morrowind expansion and all of the content in the vanilla experience.
ESO's "Blackwood" DLC, which includes the Blackwood, Greymoor, Elsweyr, Summerset, and Morrowind expansions in addition to the base game, has also been significantly discounted to $16.49 (from its usual price of $50). That's a pretty decent value. As an ESO fan myself, that's probably the pack I'd recommend to new players, provided they enjoy what the trial has to offer.
Zenimax Online Studios is using this F2P week as an opportunity to hook new players into its upcoming expansion, known as "High Isle," which launches on June 6. High Isle will take players back to the land of the Bretons and the "seat of chivalric culture" as they seek to confront a new evil known as the "Ascendant Lord." This expansion can be pre-ordered for $40 right now, though we'd obviously recommend trying the base game before purchasing extra content.