I sell a laptop battery meter (http://batterybarpro.com). It's not income replacing; it makes about $1,000 per month, but it's been crucial in saving enough for down payments one two houses.
I've tried to get the revenue numbers up, but I've never been able to break a $2,000 month.
I did. It used to be $10. I've tried a whole slew of different pricing options, like cheap, but time-limited, as well as price points between $4 and $10. I've found that for a simple tool, people seem to find $10 a bit off-putting. I've been running a 50% promotion recently (for over a month now) and revenue is up about 70%.
The lifehacker article came out in my second month of sales. I went from $400 in my first month to $1800 in my second month. Unfortunately, that's pretty much been my best month. I do get quite a few people coming in from web forums and other review sites though.
Maybe you might consider adding at least a form for "Looking for the Mac version? Enter your email to get a notification when BatteryBar Pro is released on it."?
Then if eventually you release that version, mailing that list could make quite a bit of money for very little effort now. If the list grows very large, it would add further justification to develop that version too.
I've tried to get the revenue numbers up, but I've never been able to break a $2,000 month.