There are times when I consider launching my own company again, most often when it is late at night and the inpetitude of so many other companies gets me too worked up. Then I sit back and think about details and write it off.
I've worked for myself in the past a couple of times, and each time it was both more fun and more difficult than expected. Getting a couple of clients is usually easy, getting a ten more is common, but getting "many" is hard and getting "lots" is something I've never done - lots of users for free sites though, along with the associated support burdon!
So the though dies away once I sit down and work out the net profit I'd need to live. My expenses are low, so let us pretend I can easily live on £1000 a month. So the "company" has to make more than that, to cover costs, but perhaps not much.
Pretend you were offering DNS hosting you'd probably be able to implement that easily on, say 10, virtual machines, net of £150 a month. Imagine clients pay £5 for an unlimited number of domains that means you need to have 1000+150/5 = 230 clients. Not impossible, but also not easy.
Pretend instead you're offering backup space, and the numbers get bigger because disk is expensive. Again getting some users would be easy, but getting lots would be hard because your competition is dropbox, skydrive, etc, etc.
Once you start thinking of "ideas" they come easily, but the hard part is being realistic about what people would pay for. As always the idea is the easy part, the execution is the hardest part. Realistically if I were to be desperate to work for myself at short notic I'd do the obvious thing - I'd buy a pair of ladders, a bucket, and clean windows. Low overheads, reasonable demand, and I'd be both "fit" and "outdoors".
When it comes to paying for online services off the top of my head I personally pay for maybe two things, both of them niche (although profitable for their providers I'm sure), and I know many people who live on the internet but pay for nothing.
For example I'm a VIP member of an online modeling community, which in theory allows me a higher chance of persuading interesting people to pose for me.
In practice the turnover on those sites is immense. Lots of cute boys and girls hear constantly "You're so pretty, you should be a model", which is true in perhaps 1% of cases, and the net result is you have a few hard working people who do good things day in day out, and many flighty teenagers who'll pose for two-three people, and then never do it again because they realise it is neither glamourous nor easy money.
Two things I've semi-serously considered recently where hosted "status pages", and hosted "domain parking", but both have many competitors and both I can see a) some people would pay for but b) not very many.
I suspect there is no universal "I'd pay for this" online service hwich is both competition free and genuinely trivial to setup, but I'd be curious to see what people are missing, and even more curious to see what people do pay for.