We released an expansion card for Raspberry Pi packed with resources:
Eight on-board relays, 8 12-bit ADC channels,12-bit DAC output, 8 opto-isolated inputs, 4 open collector outputs, 6 GPIO's.
The card is four-level stackable. Tentative price is similar with RPI, but we have it currently on Kickstarter for only $25.
Relays are spec-ed at 10A/250V, but the board's target is Home Automation, limited to 2.5A 24V.
A short video description is also on Youtube.com.
The card is not technically a HAT, because it exceeds the form factor, but it meets all the other HAT specifications.
Up to four cards can be stacked on top of each other: