The most popular replacement, and the one I personally use, is Feedly. http://feedly.com/
fwiw i switched from Google Reader to https://bazqux.com/ - The UX is very similar, has no problem dealing with a lot of feeds and is fast enough to keep up with my ~1000 item/day habit.
https://www.inoreader.com/ - I might be mentioning it too often, but it's a great replacement that mirrors the Google Reader UI closely while adding a lot of enhancements. It also doesn't throw away old cached posts.
Feedbin is a pretty good alternative, especially when coupled with Press as an Android client.
Another alternative: theoldreader.com - they aim for a very close replica of the original.
I like Newsblur
I second this. I tried an awefull lot of feed readers when Google announced the end of Reader, and Newsblur is by far the most usable one, and it is actually pretty good. I would stay on Newsblur rather than returning to Google Reader would it come back to life.