One of the things most cultures have in common, especially earth centered ones, is a reverence for the ancestors and Judaism is no exception. Numerous times in liturgy Abraham, Isaac and Jacob are brought to mind as are the Imahot, Sarah, Rivkah, Rachel and Leah. These, among others, are invited into the sukkah and we are invited every Pesach to join in spirit with our ancestors who witnessed the plagues and the parting of the sea by being reminded that these things were not just done for them but for us as well.
For those with greater sensitivity it is a short step from intellectualy recalling the Ancestors to actually feeling their presence. In the invocation to my Rosh Chodesh ritual I invoke the Foremothers and most of the time I actually feel their presence. One of the greatest experiences I had was at my wedding where I felt the sprits of the Ancestors attend almost as if a nomadic tribe had come to celebrate with us complete with their goats and all that would have traveled with them. This should have been of little surprise since the opening blessing the rabbi did was an invitation to the Ancestors. It was thrilling especially since my dream was to have my spiritual community join in celebrating with my beloved and myself.
The Ancestors aren't the only spiritual beings I've experienced. Even back in the days when I was a good Catholic girl I had expereinces where I'd be praying alone in an empty church and I'd feel a presence or 2 in specific places in the church. Then there are the times I've been leading synagogue services and standing on the bimah I have felt myself flanked by 2 presences of the same nature; these I believe are angels. Why I should be so graced I don't know - it's not like I'm extra virtuous or anything but Judaism believes that there are angels and spirits all around us so I guess it shouldn't surprise me that they would join me in prayer.