It doesn't really count as a giveaway since it was more of a support incentive, but I mailed 4"x6" art prints to people who donated 5k ink during the last inksgiving. The only cost was buying stamps, but the overall support I received paid for itself.
Printing stickers can also be pretty cheap and you can fit those in an envelope too.
I see dozens of art raffles daily on my Twitter, so if you don't want to spend any money, I'd say they're pretty effective in gaining new followers. (How many of those actually stick around after the raffle ends is another story, but any growth is better than none imo)
I'm debating on doing a merch or art giveaway next. I think merch is good for promoting your comic since the readers are already familiar with your characters, while art giveaways are good for attracting mutual artists (or people who just like free art).