I really like Kaleidoscope and have been using it for quite some time now, since it first launched on the MAS. It's a very pricey but I think it's worth it. It'

I really like Kaleidoscope and have been using it for quite some time now, since it first launched on the MAS. It's a very pricey but I think it's worth it. It's a great program, looks good, makes git diffs really easy and launches in a second on my Mac. It also provides integration with services like git so you can just do git difftool and have Kaleidoscope launch with the diff between the local and remote. For big hairy merges, this is a lifesaver! I also use vimdiff sometimes and it's pretty great as well, with the advantage of editing a file inside vim.