For inter-galactic travel by:
Bussard ramjet, see here;
T-machine, see here;
many instantaneous space jumps, see here.
I made a link to the "In Space" post because it imparts information on space in the Cloud Universe. However, it also refers to asteroidal, interstellar and inter-galactic space so I considered it appropriate to add further links to posts covering these aspects of space.
On the issue of inter-galactic travel, as with the end of the universe (see here), AI (see here), future histories (see here) and time travel (see here), Poul Anderson once again seems systematically to address every possible aspect of the topic. Bussard ramjets and T-machines are theoretically possible now. Although, in World Without Stars, the characters visit only a single planetary system in inter-galactic space, instantaneous jumps combined with the antithanatic suggest the intriguing possibilities of visiting every galaxy and of traveling to the relativistic edge of the universe if it has one.
Similarly, in There Will Be Time, two concepts creatively combine: STL + time travel = FTL.