Combo: Workshop and Gardens
|Workshop / Gardens|
Workshop and Gardens is one of the earliest combos discovered for Dominion. Workshop is used to gain Gardens, and increase the number of cards you can acquire per turn, which boosts the value of your gardens.
While Workshop is a decent Gardens enabler, particularly within the Base set, later expansions have provided much better combos using similar cards. Ironworks in particular stands out as it gives a bonus for each card gained, and can non-terminally empty its own pile.
This combo is fairly simple - open Workshop/Silver or Workshop/Workshop, and whenever you have a Workshop in hand, gain a Gardens with it until the Gardens pile is gone, then switch to any other pile. This a rush strategy, so your goal is to empty three piles - most often Gardens, Workshops, and Estates. The Workshops allow you to keep gaining moderately-costed cards even when your deck becomes mostly Victory cards. In a non-mirror, it is actually optimal to load up on workshops before starting on gardens. Simulations indicate that it is optimal to gain 9 workshops before switching.
The Ironworks version of this tends to be quicker - gaining Victory cards draws you another card, which can boost your buy for the turn, and Ironworks can't collide (unlike Workshop): the first Ironworks in your hand gain Actions, and the final one can gain a Victory card.