Anything from Amember to Quickbooks is fair game here. In the hosting industry, iDevAffiliate and Plesk Billing were the pieces of software we would have paid a lot of money to have someone else deal with. The more niche and more complex/annoying/frustrating the software, the better! Even if the software company offers free installation, you can make hundreds or thousands of dollars training business owners or their employees on its usage. Focus on one piece of software and become the trusted expert. Create videos and tutorials using screencast software like Camtasia to increase your profitability; this may also lead into a niche product that you can sell. (Amember offers “free installation”, but it took my boyfriend and I over 10 hours to configure it properly. That’s definitely something I would outsource next time!)
One of the cool things about Google AdSense is that it's so easy to get set up. If you have a blog or website, you can sign up for a free Google AdSense Account. From there, Google will give you a unique code that you will paste onto your website. Google takes it from there, tracking your page views, traffic, and earnings on your behalf. There is no upkeep or maintenance to get this thing going, which makes it a no-brainer if you have a website already.