I've recently had cause to refresh everything installed onto my Macbook Pro, which got me thinking about the applications I love and use the most. The first five apps I immediately download and install onto any computer I have to work on are:
- Jumpcut is a super minimalist clipboard buffering application that I've been using since 2003. It's easily my favorite application of all time, as it's saved my ass multiple times a day every day since I first installed it. You should get it!
- Alfred lets you easily launch applications and search your computer or the web for anything. I used to use Quicksilver for this, but I've found that Alfred does the trick quicker and better.
- Chrome is Google's browser, which I use because it loads pages faster than any other browser, lets you search in the address bar, and perhaps most importantly for my needs, it can handle having a gazillion tabs open in multiple windows at the same time for months at a time without constantly crashing on me like both Safari and Firefox are prone to doing.
- Postbox Express is my favorite mail client , which says a lot since I've been dissatisfied by every single OS X option available to me since I left Windows—and Pegasus Mail!—behind over a decade ago. I use privately-hosted IMAP for my primary email (instead of Gmail like most people do) partly because I'm paranoid and don't want my entire life living on Google's servers, and partly because I find myself far more productive with a desktop email interface. It just does everything I want it to do.
- Meteorologist lives in the menu bar and keeps you updated on current and future weather; it balances having a stripped-down view and being fully-featured better than all of the other current menu bar weather applications . I walk a LOT, and like being appropriately dressed for whatever temperature it is outside, so this app is key for me!
- Fraise is a super simple text editor. Everyone needs one, even if you don't work with many text files and so don't think you do, and this is the one I like best. 
A few other applications I use and love: the Last.fm scrobbler, for tracking the music I've been listening to over the past few years; Flickr Uploadr, for when I have a huge bunch of photos to upload to Flickr; VLC, the best media player for OS X; Transmission, the best torrent client; Paparazzi, for taking screenshots of webpages; Dock Dodger, for disabling dock icons of apps that are assholes about not letting you choose to keep them in the menu bar; Dockdrop lets you drop files onto its icon and immediately uploads them via ftp/webdav/Flickr; and everyone's favorite, Dropbox, for syncing shared folders to the cloud. What apps do you recommend?
 I'm sure Postbox 3 is fantastic, and I'd be more than happy to pay for an upgrade to it in the future if I need to, but Postbox Express currently meets all my needs.
 I switched to Meteorologist after WeatherDock 2, which I loved, stopped working, and might eventually switch to WeatherDock 3 when it eventually comes out if it's like the previous version.
 Fraise is based on version two of Smultron, before the latter's developer first stopped developing it and disappeared, and then reappeared, closed source, and started selling it. I might consider switching to Smultron 4 once I upgrade to Lion.