What is 'that' book?

I cannot be the only one who remembers reading a book years ago and is unable to remember the title of it, so I figured I would start a thread with my missing title in the hopes that someone could figure it out and a place for other people with the same issue to broach the topic.

Alright.

The book in question involved people from another dimension, I think they were humans, who developed the ability to go to alternate dimension. They did this for a while, conquering world after world, until they came to ours, where it ended up that we were 'giants' compared to them. So we killed them and took their technology and set about colonizing the multi-verse.