As a former member of a US glider club (I live in Europe now, and it's all winch launches), I always thought being a tow pilot was a good way to build time. Can a private pilot tow gliders in exchange for money or credit with the club, or is a commercial ticket needed?
A private pilot certificate is the minimum required. In addition you need the following experience and endorsements to be current:
CFR, part 61, section 61.69, outlines the requirements for towing privileges. You must:
1) Hold at least a private pilot certificate with the appropriate category rating;
2) Have logged a minimum of 100 hours as pilot in command in the same category aircraft used for towing;
3) Have a logbook endorsement from an authorized instructor certifying you have received ground and flight training in gliders or unpowered ultralight vehicles, and is proficient in the areas listed in part 61.69(a)(3)(i)(ii)(iii) and (iv);
4) Have a logbook endorsement from a pilot that already meets the requirements of part 61.69 (c) and (d), who has accompanied the pilot on three flights which has certified them to have accomplished at least three flights in an aircraft while towing a glider or unpowered ultralight vehicle, or while simulating towing flight procedures; and In the preceding 12 months has performed three actual or simulated tows accompanied by a qualified pilot or has been towed for three flights in a glider or unpowered ultralight vehicle. In accordance with part 61.52, pilots towing under an ultralight exemption may credit experience obtained in ultralight vehicles for their towing experience and endorsements.
Added per comment on question above:
Yes, a private pilot can be compensated for glider towing.