I'd like to solve tsumego online, not having to install a program or app locally. Are there any sites that allow this? How about for smartphones?
The traditional problem site. Has many user contributed problems. Personally I'm not too fond of it, since the problems often vary in style getting me out of the problem solving flow.
Currently my favorite. Has several large collections of problems. I like it for mass solving relatively simple problem.
First you need to login with a google account. Then choose a collection on the right. And finally you need to switch to the Question or Progress tab on the right to solve problems.
This is my favorite: http://www.hitachi.co.jp/Sp/tsumego/index-e.html - It's especially great for anyone between mid-SDK and dan level.