A good dot com may be difficult to find now. But there are a lot of gTLDs and ccTLDs from the .info to .eu to .tv to .co.in and they all present opportunities being discussed in several good forums. Put your thinking hat on, buy a famous word domain for a few dollars and put it up on the domain selling sites.
Lost your thinking hat? Hang out at SEDO. DNForurms, Afternics and other places where domains are bought and sold. Provided you learn enough about the market to recognise bargains you could make a living from just buying existing domains and reselling them.
If you're smart you'll run dictionary checks against available domain names and auto-check them against search volumes (using OST, Wordtracker etc.) for that term and Pay Per Click (PPC) rates in the major ad networks (example) to work out which ones are likely to be more profitable. If you can pick up the domain for a profitable term that's searched for often you can use a domain parking program. Or post a little bit of relevant content and get a link or two ... and the search engines will start sending you traffic. If the phrase people are typing in coincides exactly with your domain name it gives you a great head start with SEs.
If you're smart AND a linguist you'd be doing that in multiple languages. And not paying for any domain till you've tried it free for five days to see if it does indeed get any type-in traffic (and how well that traffic converts). After you've tried it for five days and dropped it there's nothing to stop you immediately picking it up again for another five day trial. Strange, but true. It's not kiting, it's legal.
Misspellings. Massive opportunities still exist in the misspelling/typo market. People trying to get elsewhere land on your site instead ... and you sell them stuff (or use the domain parking idea). Some even tempt fate by making PPC opportunties out of typosquatting on trademarks. Finding typos has never been easier. There are many tools that will find misspellings for you. How easy can it get?
Domain parking and type-in traffic: People sometimes guess at URLs. If they want a plumber they may try plumber.com though they've never used that site themselves. Find terms that people may be typing in (I will provide a detailed guide to this when I get a chance), buy the domain and populate it with ads. There are several ad programs to monetise your parked domains. Or combine this with the previous idea to buy plummer.com or similar typo domains to make money online.
Drop catches. People sometimes forget to renew their domains and these expire. Picking them up will give you some remnant traffic from sites that link to this domain/people who've bookmarked it etc. In some cases the traffic can be pretty high. Provided you're fast enough to replace the copyrighted content that was there with something else you can make quite a profitable business from doing nothing else but this.
A variation on the above. Sell the domain back to the previous owners. Note that you may want to tread carefully and get familiar with the rules for that TLD before you start sending off ransom notes. For example, with ICANN (domains that end in .com, for example) the moment you send the previous owners an email saying you've got their domain and you'll give it back for $10K... you've lost.
It can't look like a ransom demand. Be reasonable and read the small print of the UDRP. No UDRP required if you're sitting hostage on myspace.com/theircompanyname or the equivalent at blogger, mybloglog, or other big destinations. LOL, watch them kick themselves and sack their web advisors who told them about taking the "dot info" but omitted to mention the importance of protecting the brand by owning the associated myspace directory (and others)! And it costs you nothing!
Run a domain management service. Hundreds of thousands of webmasters (or more) have a large portfolio of domains. A lot of them would like the boring bit taken out of their domain management. You can run their DNSes or just a service reminding them when each domain comes up for renewal. Or an automated monitoring service to tell them when one of their domains/sites is inaccessible.
Start a directory to list domains for sale. That's what people like SEDO do. You can get money just for allowing domains to be listed in your directory.
If you're running a service putting buyers and sellers of domains/sites in touch with one another you could get money for add on services (like providing escrow facilities). For ideas have a look at what existing domain intermediaries offer.
Run a domain research service. Wonder what happens when a manufacturer is looking to name a new model car? Or starting a new range of clothing? They need trademark and patent research but now they also need some domain research. Which of the literally thousands of combinations and misspellings (+
Start your own country: Whoa! yes, you're reading it right. If you've heard of Sealand (what is Sealand) you'll know that starting your own country is not that far fetched. Once you have your own WhackyCountry you can apply for a .wc (yuk) TLD. Sell millions of domains. Keep some for yourself. Ever wanted a Google.___?
Perform domain services for businesses and then send them a proforma (even if they've never heard of you). Explain that it's free this time but you'll gladly keep acting for them for a small fee. For example, there are thousands of big businesses whose half-wit webmasters/ developers didn't put in a redirect from the non-www to the www versions of their sites (or vice-versa). One entrepreneur made a few thousands just from pointing out to businesses how they were losing hundreds of customers every year who were landing on http://xxx-companyname.com and finding nothing there.