I can also vouch for lichess.org

It's free software (https://github.com/lichess-org), and you only need web browser to play [1]. Even Magnus Carlsen and many other top Grandmasters play there.

As for cheating, all games are checked for computer play, so even if you can occasionally lose a game or two from a cheater, he will be caught, and you will get back your rating points (if you care).

[1] Dunno what they are using for latency adjustments on lichess, but it seems to work fine. No external software necessary, unlike chess servers of yore: https://www.freechess.org/Help/HelpFiles/timeseal.html

I highly recommend Chess Tempo for tactics training. A large database of problems drawn from grandmaster games, insightful user commentary per problem, and a full classification of each problem by the type of tactic e.g. attraction.

chesstempo is neat because they have ELO-like ratings for tactics puzzles. It's the first place I saw such a thing.

Alot of those puzzles are damn hard, ELO ratings there are legitimate.

I feel that chess Tempo has really given me tools to recognize recurring tactical motifs in actual play.