(No real order)
1. PEPFAR Initiative to combat HIV/AIDs (as multiple people have mentioned)
2. The PATRIOT Act
3. His response after 9/11; his call for unity in the aftermath of 9/11 (also includes his speech at Ground Zero)
4. The removal of Saddam Hussein (there can be debate on whether or not it was the right decision to do so, among other things, but Hussein's ouster is most certainly a major part of his presidency)
5. passage of Medicare Part D
6. Keeping America safe after 9/11 (with no terror attacks occurring on U.S. soil during the remainder of his term)
7. Partial Birth Abortion Ban
8. U.S. increased sanctions on an increasingly erratic Iran during the Bush Presidency (UN Res's: 1696, 1737, 1747, 1803)
9. Reorganizing U.S. intelligence agencies and creating the Department of Homeland Security
10. Substantively increasing U.S. presence and aid to African countries
11. Boosting and doubling the research budget of the National Institute of Health
12. President's Malaria Initiative
13. Proliferation Security Initiative (103 Nations endorsed it as of June 2015)
14. Bush Tax Cuts
15. Condemnation of Russia for it's Georgia invasion; Signing SORT w/ Russia
16. Increasing foreign aid
17. TARP Bailout
18. Paying off US debt to UN
There are others, of course, but these are just some of them.
I voted for keeping America safe post-9/11