#1 - Blogging
A good place to start considering I have a detailed understanding of how this site generates money. Seopher.com started as an idle platform for me to express the more coherant of my thoughts, but around October I started spending more and more time on it and it started to blossom. Funnily enough, the website has started to generate money.
Google Adsense is the biggest winner for me because it's really the only avenue I use to monetize the site. I don't like/want overly invasive adverts so I don't tweak my Adsense much. However, through is avenue I've managed in excess of $100 a month before which isn't bad considering money isn't my primary objective.
However, if you do wish to make money online - I cannot recommend JohnChow's blog enough. He's a self proclaimed dot-com mogul and he is able to bring in a full-time income with his blog.
There is sufficient on his site to motivate and advise you into making small bits of money.
Smart and effective manipulation of social bookmarking/news sites like Digg, Reddit and StumbleUpon can bring high levels of traffic; this can result in higher advert clicks, better traffic rankings (on sites like Alexa) and someone may wish to pay you to reprint your content - all of these aspects help improve your online profitability.
Really, it is possible to make acceptable levels of money purely from your blog. Sell text links (Text-Link-Ads.com), use Google Adsense and any other number of advertising solutions (just see John Chow's site for more info). It can be done and it's not *that* hard to make small amounts of money on the side.
#2 - A fad that goes viral
Silly little ideas that are good for a giggle are another way to make money online. Something that's quite cool but has no practical application just needs one major resource - an idea. If you can develop something quite cool that people will enjoy once or twice and pass on to others then you're on to a winner (provided it's monetised). A slightly daft but cute idea that you can turn-around quite quickly (in terms of development) could bring in enough money to make you smile; nothing life changing though.
#3 - A proper web2.0 startup that gets bought by Google
I hate using the term "web2.0" but I have to for this category. Youtube was bought by Google for $1.65 billion obviously made the creators smile. Feedburner was recently bought for $100 million. Google also bought Writely for a sum that I'm uncertain of too... So if you develop a good product, get an excellent user base then you're in a nice position for Google to write you a cheque.
#4 - Be an expert in your field, get recognised and make appearances
If you have a blog within a field that you're an expert in, raising awareness of yourself (with some shameless self promotion) might mean that people start to care about what you think. It's not a massive step from *here* to being paid to speak at events. You can make a *fortune* because of your online representation (as a person) so that's always another angle to take.
#5 - Be an expert in your field and offer consultancy
As the title says really (and overlaps with #4); if people really care what you think they may well offer to pay you for your opinion.
#6 - Write a book
While some of you may raise an eyebrow to "write a book" as a piece of advice for "making money online" do bare with me. The site Lulu allows you to write a book and send it to them for printing. You write a book, save it as a PDF (along with artwork) and submit it to them: they do on-demand book printing. You get your own ISBN number and they then stock your book in the shop should anyone else want to buy it (also offering a "download only" version). You set your own pricing and there are no start-up costs. You could even just write one book, get it printed and put it on your shelf. However, why not write a book, get it published and then try and sell more copies? Nice and easy with Lulu.
#7 - Get friendly with Affiliate schemes
A time honoured strategy; have a site for "the best deals on the Internet", put up "bargains" that refer visitors to a site using your affiliate code. If they buy a product or service then you get a percentage cut. Many affiliate schemes now use 30-day tracking codes meaning if the user came to their site with your code in the URL; a cookie gets saved on their machine. If they make a purchase from that vendor within 30 days and the cookie is still present - you get a cut. Make a popular site and you'll send hundreds and thousands of people away with your tracking codes in place. Big sites such as Amazon have affiliate schemes so they really are an excellent way of making money.
#8 - Make something and sell it
it's possible to make a product and sell it given the landscape of the Internet in 2007. I'm not 100% certain whether it's as viable as it once was but you can set up an online shop very easily these days (using any number of pre-built packages) but implementing Paypal merchant tools isn't too difficult either. If you have a good idea - the climate of the Internet today is still allowing you to sell it online (marketing is your biggest problem).
That's all I can think of at this moment in time, 8 good ways to make money online in a relatively short timescale. If you can think of any more and have good examples please do let me know and I'll make updates. Most of these ideas can be done quite quickly; this site has only been done on a regular basis since October and already it's self funding. So I'm an acceptable example that you can make in excess of $1000 within 6 months of starting a website so someone with some proper skills and knowledge could probably do a better job.