I'm currently a Mac/iPhone user, and I have some applications that I have found extremely useful over time. For those new to these platforms, I thought I would share my favorites:
Adium (Mac): really nice IM aggregation client. Beautiful, extensible interface. Wish it did video ...
Blogo (Mac): my new favorite blogging client, which has taken over from the venerable MarsEdit. Love the Twitter integration. Blogo's microblog view has replaced Twitterific for me.
Skype (Mac): combined with my Audio-Technica AT2020 mic and my CEntrance Microport Pro USB preamp, I sound like I'm doing phone calls in a recording studio. It's awesome.
Facebook (iPhone): very nicely implemented version -- just what you need in the mobile app.
Twitterific (iPhone): great little microblog browser on the phone. Particularly speedy at loading linked sites.
BlogPress (iPhone): since I am still on Blogger for the time being, this is the best iPhone blogging tool I've found. If I ultimately go to Wordpress, they have a nice looking iPhone app. Both let you post iPhone photos to your blog.
Evernote (Mac & iPhone): billed as a memory extension tool, this is really simple but profound. Easier to see than describe, I use it to keep track of all those little bits of info that you end up looking up ten times a year (i.e., opening times for the zoo). Highly recommended, particularly given the Mac's awesome ability to print anything to a PDF.
OmniFocus (Mac & iPhone): the Swiss Army Knife of GTD.
Quicksilver (Mac): I'm still figuring out all the cool things that this can do. Basically, it runs as a background process on your desktop, and can be used to quickly launch apps. But that's like 1% of what you can actually do with it. As close to geek magic as software gets.
JungleDisk (Mac): a really nice front end to Amazon Web Services S3. Storage in the cloud, represented as a drive on your desktop.
MobileFiles (iPhone): if you use the iDisk feature of Mobile Me, this rules. Let's you access your iDisk in the cloud from your phone, view files, download, etc. The upcoming pro version will include viewers for the Microsoft Office suite.
1Password (Mac & iPhone): the Ferrari of password vaults. Really, more of an identity management solution. Integrates with your browsers on the desktop (although not on the iPhone). Generates strong passwords that you don't have to remember. Fills out forms. If you do a lot of login/ecommerce, this is a must have.
Media & Entertainment
NetNewsWire (Mac & iPhone): my all-time favorite news reader and feed aggregator. Syncs to cloud so you can use it on multiple devices and not miss anything. Nice interface, even on iPhone.
Yelp & OpenTable (Web & iPhone): two great tastes that go great together. The Yelp iPhone app is not as sexy as UrbanSpoon, but it has better reviews. OpenTable's iPhone implementation is great, and it's one of the most useful services around.
Chroma (Mac): very pretty movie player that supports avi's as well as quicktime and DivX. Has some cool movie-specific features. Also nice substitute for the built-in DVD player.
TextMate (Mac): an integrated development environment masquerading as a text editor. An essential tool for coding Rails. Highly recommended.
CSSEdit (Mac): an amazing single-purpose product for editing cascading style sheets, the visual language of the web.
Sequel Pro (Mac): the new name for the wonderful CocoaMysql code base. A powerful, easy-to-use front-end for Mysql databases.