Your basic reference is IRS Publication 526, Charitable Contributions, and the juicy stuff starts at the bottom of page 4. "The value of your time", nope.
Deductability applies to things which are entirely, or provably mainly, about your volunteer work, and not for other non-charitable purposes. Dining doesn't count against you, you have to eat.
- Your commute to the Wikipedia office is tax deductible, as documentable transit fares, tolls, parking, and mileage.**
- Special equipment such as drone, ammo, work boots, nomex suit etc. that you only use for charitable purposes are tax deductible. For instance if you use a chainsaw only to edit Wikipedia and help a local school cut brush, then it is tax deductible along with bar oil, spark plugs, repairs etc.
- A case could be made for deducting part of your home (e.g. home office) if you only use it for charitable work. Be ready to firmly defend: a garage used entirely for long-term storage of Wikipedia equipment, fine; the home office with spiff gaming PC "used only for Wikipedia", yeah right.
- Meals only when you travel far from home and stay there overnight.
- Travel expenses when you can prove the purpose is almost entirely charitable, and the "stink test" applies very strongly here. Going to New Orleans the week after Katrina is an easy sell. Wikipedia sorely needs better pictures of celebrities, so if you spend 2 weeks in L.A. collecting them, you better be able to show that it occupied nearly all of your time, and doesn't smell like a general vacation.
- Socializing with volunteers (while not doing volunteer work) doesn't count against the travel being charitable, but it doesn't count for it either. If volunteers work 10-4 then dine and socialize til midnight, that's fine. If it's all social, then nope. See "conventions" in Pub. 526.
- Travel that is superfluous (not needed for the volunteer task), nope.
- "The value of your time" - no.
Your tax deduction is capped at 50% AGI, but can roll forward up to 3 years, but is immediately lost if you do not itemize.
** Mileage is EITHER actual gasoline and oil OR 14 cents a mile. "Documentable" doesn't mean you need to fastidiously save receipts. Just log your trips and claim expenses that are known quantities (e.g. transit fare is $3.90 or Google Maps says the roundtrip is 100 miles so $14.) The gas and oil exception is if you are driving a very large or expensive vehicle. The 14 cents is intended to include wear and tear. IRS hasn't checked the price of gas in the last 20 years.