A fellow HOLer mentioned a good site for Dominion, which I found was a great way to explore the expansions rather inexpensively. A few months back, I played against the bots quite regularly, although where possible I played against human players. While the bots were great, I was able to figure out some of their patterns and adapt my strategy accordingly (I also noticed that there was one particular combo that the bots didn't use when they had the necessary cards in their hand, so those games ended up being rather one-sided).
Steam currently has a sale on their digital boardgames (ending 6 August), which I'm very tempted to take advantage of. There's some great titles on offer, including Race for the Galaxy, Scythe, Splendor - as well as Tabletop Simulator, which in turn has additional discounted titles associated with it that can be bought.
I've also been looking into accessible options for playing boardgames online (apart from HOL's Gathering and Games, that is

Do any of you play boardgames online?