Consider donating to a pro-choice organization. A few of my favorites:
The National Network of Abortion Funds (list of local funds here)
The National Abortion Federation
Medical Students for Choice
Planned Parenthood
Haven Coalition
Leave other suggestions in the comments.
UPDATED to add this press release from the National Network of Abortion Funds:
The Network has also received many requests from women who received care from Dr. Tiller and from activists in the reproductive justice community to set up a Fund in Dr. Tiller’s name. In response, we have established the George Tiller Memorial Abortion Fund to assist the women to whom George Tiller dedicated his life. The Fund will assist women in the second trimester to pay for abortion care, as well as pay for travel and lodging en route to providers. To donate to the Fund in Dr. Tiller’s name, please send contributions to:
George Tiller Memorial Abortion Fund
c/o National Network of Abortion Funds
42 Seaverns Ave.
Boston, MA 02130
You may also donate online at http://www.nnaf.org/tiller.html