Mac: Appalicious is the front end of a database for searching, sorting and filtering items in the Mac App Store. It's especially great for finding free and on-sale apps, and additional features like wish lists, version and price histories, and the ability to save search and sort combinations help you easily zero in on exactly the app you're looking for.
Once you've found the app you wish to purchase, one click takes you to the page in the Mac App Store for purchase and download. All of the great features of Appalicious come at price — $US12.95/year, but you get a hassle-free seven-day trial before having to decide whether you need the service or not and the trial does not require credit card information, just an email address. You can also use a referral code to gain one free month for each person who starts a trial of the service. (my code is PDDM-FNTC-CQNF-ZZSH if you're interested)
If you rarely purchase apps from the Mac App Store Appalicious most likely won't be worth it for you, but if you're a heavy user and/or a bargain hunter you'll quickly make up the $US12.95 cost.