I would love to build something this weekend. Something simple, built upon API's of other application or simple hack. Ex: http://snapsite.me/demo.php, http://www.ngotha.com/ I would love to hear ideas from you. Definitely it should be helpful to other followers of HN.
Always had this idea: an app that notifies you when someone responds to a post on a forum. For example, if I'm looking to hire a freelancer and I post on 10 forums I don't want to bookmark and check each one but I also don't want to get email notifications everyone either.
Good timing, just had a SO question that fits a weekend project:
Implement social share count API in Scala / Java: http://stackoverflow.com/q/12847138/239168
I would do it myself but my wife and 2 small kids will object.
A less interesting idea(that can make you some money) - Hack a Wordpress theme using http://underscores.me/ and Twitter Bootstrap. I did it as a weekend project last sat/sun.
Another idea :
Knowledge Explorer : Write a scraper that crawls sites like HN/Reddit/StackOverflow/Quora etc and organizes articles/questions/discussions etc in categories like "Python", "Ruby", "UX", "Machine Learning". Allow users to explore it according to these categories and sort it on basis of votes/comments/answers/etc. It will involve web scraping and machine learning. I need it personally.