I'm asked this question often, mostly in regards to my phone (android), but I also purchase a far amount of apps for my computer as well. Here are the last 5 apps that I've purchased...
Clean My Mac - my most recent purchase. More out of desperation than desire. It's the best. Expensive. But according to MacWorld it's rated 4 and 1/2 mice. MacUpdate also thinks it's pretty decent too with a much more traditional 4 stars. The reason I sprang for this software was because when it comes to "cleaning" my computer I take it seriously. Like, I seriously don't want the wrong things cleaned off of my computer.
By the way, my total "recovered" thus far is 4+ gb.
RipIt - I use this for ripping my dvds on to my computer. My goal is to eliminate my dvd collection and instead have it all on a computer. Less clutter, less hassle, and way easier to search, especially when wanting to watch certain tv episodes. I've found RipIt to be SUPER easy. I'll specify is where I want to play it when I'm done and it determines what format it needs to be in. Example: I say I want to play it on itunes it puts it in the appropriate itunes format. Or if I say I want to play it on my PS3 then it puts it in the correct format for that one. I love this app. Seriously.
Tagalicious - My music collection would not be the same - even close to the same - without this. This little sucker can identify (magically) all those "untitled albums" I had lurking in my itunes library. I had no idea how much more music I had until I got it all tagged, which wasn't that hard by the way. It has a great user interface which always makes me feel more confident in an application. This also inadvertently helped me find many duplicate songs I didn't know existed.
Rowmote - This is an app for my iPod Touch (that I got free with my Mac - love being a student). I use to to extend the power of the Apple Remote app to the other applications on my computer - at the time, Hulu Desktop. It's a nice little app that let's me control my computer from my couch. Now, I'll also admit that I had an itunes gift card, which also influenced the purchase of this. Not sure if I would have purchased it otherwise, but I've found it helpful. Also, I sang "Row You're Mote" (to the tune of Row You're Boat the entire time it took me to find and upload this picture).
Chronories - This was my very first application I purchased on my new Mac. It automates my journaling. It also has little flashing things go off in my tool bar to remind me to write a bit or snap a picture or rate my mood. I recommend checking out their website and checking out their video. It would take me a long time to explain why I chose to spend $29 on this. For me, someone who is a tad forgetful, seems to always be going at the speed of light, but wants to have a record of my life, it's perfect.
Looks like I lied about not updating soon in my last post. I've learned I can't predict when I get in my posting modes. When the next mood arises I'll put together a post on my free apps that I use. There's a few more of those, get a excited. While you wait I suggest checking out LifeHacker if you haven't already. Love reading it.