ERM software. When a new client comes onboard, install the product and database, but make sure not to make any optimizations, not even the basic indexes in the DB. Depending on the usage, the DB will get slower and slower in a few months. Then send a "consultant" who will simply run a script and add indexes etc and test a bit (no more than an afternoon worth of effort) and charge a high fee for it.
We had a project once for conversion (not getting into details). My team found a tool online. Boss bought it, finished the entire project in one day. Boss bills for month (which was the original estimate), puts the team to work on another project for the next 29 days, gets paid for that too. The only decent thing in this story is that we didn't know before the quote, that such a tool was available. Some would say this is smart way to do business, it felt shitty for me though.