I recently stumbled almost accidentally into roll20 and I immediately fell in love with it. I know it's been made mostly as a tool to play RPGs, but I believe it's very good even for boardgames, and even for playtesting purposes. However there are some limits which prevent me from fully using it, especially concerning card decks, so I'd like to suggest a few extra features:
1 - allow players (and GM of course) to put single cards back in the deck they came from (even better with the possibility to choose whether to put them on top of the deck, at the bottom or in a random place). Currently the only way to do that is make sure that only the wanted card is on the board and recall it as GM. Players just can't do it in any way,
2 - create for each deck a "discard pile" where cards can be added after playing them, and that can be recalled into the deck separately from other cards. Right now I can just stack used cards in a corner of the board, but then there is no way to recall them without recalling every other card on the board at the same time (feature 1 would help here as well, but for many cards it would be too time-consuming)
3 - create a deck setting allowing to choose whether the default behaviour when dragging card on the board is to play them face up or face down. Now they are face up unless SHIFT is pressed, and it is far to easy to forget to press it and spoil a game; when you realize is probably too late.
4 - create a deck setting to let GM decide if a given deck can be shuffled/recalled by players. It's probably useless for RPGs but it would be a perfect feature for boardgames, allowing them to be played even without a GM (or with the GM logged in as player only).