distributed ISP using long-range wifi and mesh networking -- technology should be just about there, and there are plenty of papers on "incentive compatible routing" so you can automatically pay people fairly for setting up efficient routers. Compete with Comcast etc.
frugality helper, fun financial planning software that helps you reduce spending, maybe through social / achievement-style means (give points for making dinner at home)
news recommendation (worked on this for a while; lots of ideas for making it work, but I needed a really good realtime news recommendation algorithm)
internet food shopping redux (I hate food shopping, and would love to do it over the internet, if it wasn't even more painful than in real life)
minipapers: for a given field (e.g., computer science systems), every day the site posts a new academic-level paper that can be read in 15 minutes by someone moderately knowledgeable in the field. So not a long paper, but one which presents a new idea in accessible form. The site is for "casual academics" -- people who like following research, but don't necessarily work in academia and don't have time to read full-length papers.
statistics for the non-statistician: most people suck at stats, but they'd like to know whether a particular conclusion can be made from some data. Give them the tools to do so (plug data in, ask question) without requiring them to know what a chi-square is or a t-test (but help them learn it if they are interested). Explain confidence intervals etc. using plain language.
calculator sharing: allow users to create simple forms which evaluate a simple formula (temperature conversion etc) and share them on a website with others. In other words, a one-dimensional spreadsheet with sharing functions built in. iPhone app would be important here.
ratebike.com: there are no good bicycle rating sites. I registered this domain intending to do something with it, and haven't yet.
webapp IDE: a full-featured html/javascript/css IDE for building webapps. Has a model of what content is static and what is generated; fills generated content with sample data; allows you to break one page into many template files; understands enough Django/Rails markup to recognize foreach loops and so on. Flashy feature is showing structure of html pages (block-level elements) in 3d, where each block is separated along Z from the previous one, so you can rotate the page and understand its structure better. CSS & JS edits take effect as you type.
Fixing technical recruiting: job postings suck; people seem to get good jobs through referrals. Recruiters make a lot of money. It seems like there is room for an app which either helped recruiters or did the job for them. Perhaps this can manage hiring flow on the company side too: tracking, sorting and archiving resumes; managing the interview/hiring process; allowing interviewers to keep and share notes on candidates.
"Greasemonkey deployed": for freelance web consultants, if you have a good prospective customer, you might want to show them a demo of your work on their website. Right now if you want to do that, you have to download their site, rehost it, and make your changes. This site would instead allow you to rehost a target site (via proxy) and insert arbitrary javascript on the rehosted version. That way the consultant can say "Here's how your website would look with my work", and it can be tested live.
writing helper: smart grammar fixing, catches & fixes most errors that people often make. has an api, so you can feed comments on your site through it. (Don't know if this is hard, but it seems like it wouldn't be too bad, especially if it were conservative.)