The free alternative is to get on StackOverflow, favorite tags relating to the language, frameworks and libraries you want to become more skilled in, then stalk

The free alternative is to get on StackOverflow, favorite tags relating to the language, frameworks and libraries you want to become more skilled in, then stalk the "interesting" tab for however much time you can devote a day. It becomes competitive quickly -- if you want accepted answers you need to be both fast and clear while either citing references or providing working example code. Grab unanswered questions off the top of the page to work on breadth of recall, grab older questions with bounties to work on more depth in more complex topics. You work on real problems and write real code, which I doubt any repetition of planned exercises can match.