Probably the most popular way to "pay it forward" in the genealogical world is to join an online lookup service in which volunteers agree to look up a record in a document, book, CD, or subscription website--or take a photo of an ancestors' grave or ancestral home, etc. Depending upon the rules of the service, volunteers agree to do these acts of kindness either for free or for only the cost they incur while doing the lookup (film developing, photocopying, postage, gas, etc.).
Most of the lookup services are arranged by location. Think not only of the records and sources that pertain to the city or county in which you live, but also those to which you have access that may relate to other places. For example, I live in Spokane, Washington, but I have many, many microfilmed records from Western Michigan counties held on permanent loan at my local Family History Center. So for my acts of kindness, I have chosen not only to take photographs of graves at Spokane cemeteries (in fair weather months, only!), but also to look up records in numerous Michigan counties, covering two distinct--and distant--locations at once.
Below are some places where you can sign up to perform a lookup. Make sure you read and understand each site's rules before agreeing to volunteer:
Ancestry/RootsWeb's Lookups Message Board and Mailing List. There are numerous other Lookup mailing lists at RootsWeb; you can browse them by entering the search term lookup* here.
Find A Grave - sign up to take grave photos at your local cemeteries. You'll need to create a free account, then go to a local cemetery's page on the website and look for photo requests under Links. Click on the photo requests link and claim one--or more--to fulfill.
Genealogy Lookup Forum
GeneaSearch's Free Genealogy Lookups
Genie-Angels' Yahoo! Group
Obituary Lookup Volunteers
iRoots.net: Community Lookup Library
Random Acts of Genealogical Kindness
Many county websites at USGenWeb and USGenNet have lookup volunteers listed. Find the U.S. county - or country (WorldGenWeb) - to which you have access to records, and e-mail the webmaster of that location's genealogy website to notify her or him of your willingness to perform lookups.
Does your home library include books and/or CDs with genealogical data in which you'd be willing to perform lookups? Books We Own, Yahoo!'s Genealogy Club Library, Lori Case's Genealogy CD List and her Privately Owned Publications are waiting to hear from you!
Cyndi's List has a long list of location-specific lookup message boards, mailing lists, websites, newsgroups, and chat rooms that would be happy to have you sign up to volunteer.