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.

Digg Reader is decent.

https://digg.com/reader

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.

For me https://www.commafeed.com comes close to the Google Reader experience.

RSS Owl (http://www.rssowl.org/) is a good replacement.