I represent a few small clients on a pro bono basis. They are both small time inventors who could not otherwise afford to have professional assistance with the prosecution of their patent applications.
Last year, I donated ~500 hours of my life to representing a client pro bono in her appeal to the Federal Circuit Court of Appeals from an adverse Merit Systems Protection Board Decision. We won over 2 of the 3 judges at oral argument, but ended up losing when the court, sua sponte, took the case up en banc. All the judges who were not at oral argument ruled against us. In total, my firm donated ~$500k in attorney time and research services to that client.
I also donate 1% of my salary to a local legal aid service organization (encouraged by the bar association, but not required).