The Doctor's saviour

In one account on the Last Great Time War, an unnamed saviour gave her life for the Eighth Doctor at the very end of the conflict: he saw her skeleton crumbling to dust right before he destroyed Gallifrey using the Moment. After doing so, he initially believed that using the Moment had made his existence fixed, preventing him from ever regenerating again; however, as he began to regenerate into the Ninth Doctor, he realised that in her last kiss, she had passed on the Restoration to him, thereby "resetting his life cycle" and saving him one last time. (PROSE: Doctor Who and the Time War)

In another account, however, these events did not take place before the War Doctor's theft of the Moment from the Omega Arsenal. The War Doctor was also not surprised when he started regenerating into the Ninth Doctor. (TV: The Day of the Doctor)

Behind the scenes
Doctor Who and the Time War never identifies the woman who helped the Doctor. Nevertheless, the idea that she passed regeneration onto him implies she is a Time Lord. Thus, she may be Romana, a character who some view to have a romantic relationship with the Doctor, which would explain the kiss.