Cheapest priced copy (Masters 25): $0.35 at Card Kingdom, near mint, their price list as of .
Act of Heroism 
Instant · Common · Hour of Devastation · #1
Untap target creature. It gets +2/+2 until end of turn and can block an additional creature this turn.
Legal inCommanderPioneerModernLegacyVintagePauper
Not legal inBrawlStandard
2 printings · $0.35 to $0.49 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Masters 25 | 1 | Common | $0.35 | $0.49 |
| Hour of Devastation | 1 | Common | $0.35 | $0.49 |
4 rulings from Wizards of the Coast
- 16 March 2018Act of Heroism can target an untapped creature. It still gets +2/+2 and can block an additional creature.
- 16 March 2018You can cast Act of Heroism even if the target creature won't be able to block right away, perhaps because you're the attacking player.
- 16 March 2018Untapping an attacking creature doesn't remove it from combat.
- 16 March 2018The effects of Act of Heroism are cumulative. If multiples resolve targeting the same creature, that creature can block that many additional creatures this turn.